hope this takes, it is list of changes since the 1990 models to present. It is from Jd website.Hope they don't mind

Check out model year changes for 9000 Series Combines since 1990. Use this information when shopping for used machines to see what best fits your operation.       Choose a Model Year 1990 Model Changes 1991 Model Changes 1992 Model Changes 1993 Model Changes 1994 Model Changes 1995 Model Changes 1996 Model Changes 1997 Model Changes 1998 Model Changes 1999 Model Changes 2000 Model Changes 2001 Model Changes 2002 Model Changes 2003 Model Changes 2004 Model Changes 2005 Model Changes 1990 Model Changes:   9400 Hp increased from 155 to 167. All 9500's and 9600 small grain combines get Hp increased to 200. Lube bank added for unloading auger turret. Metal retainers added around wood straw walker bearings Stronger straw walker crank and bearings. Chaff spreader now available. Grain tank loading auger flighting strengthened. 1991 Model Changes:   9400 power increase to 185 Hp with change from 5.9 to 6.8 liter engine. 9600 small grain combine gets 10% HP increase to 215HP. 9600 Corn combine stays at 253HP. Power boost on 9500 and 9600 gives 13 more Hp. 9500 now available with two power options, 215HP for small grain and 235HP for corn and rice. Three position throttle for 9500 and 9600. Increased cooling capacity on all models. Breather added to Posi-Torque cylinder drive. New fuel tank and filler cap for better sealing. Feeder house wear plate now factory installed. Increased drive torque of the feeder house slip clutch. Straw chopper shaft is heat treated for longer life. Higher capacity clean grain elevator and loading auger. Twenty foot unloading auger now an option for 9500. 9500 and 9501 get 10% increase in clean grain elevator capacity. The straw chopper is lowered 4 inches. Manual park brake added to left of steering column. Counterbalance weight added to knife drive on platforms. Passenger seat added as standard equipment. 1992 Model Changes:   New Generation Engines in 9500 and 9600. CTS combine is introduced. Twin tine rotor separation technology. Ideal for use in rice. New formed one-piece radiator door seal. Lower conveyor auger trough thickness increased from 1.6mm to 3.0 mm. Concave adjustment improved for more positive adjustment. Automatic speed control for corn heads and row crop heads. 9400 and 9500 get 4.75-inch wider front axle for shipping. 900 series platforms get improved pickup reel. New digital tach in corner post. Feeder house backshaft speed in digital tach. 9600 gets Hp increased to 260. Straw chopper manufacturing improved for less vibration. 1993 Model Changes:   Feeder house pivot improved. Clean grain elevator housing thickness increase from 1.2 mm to 2.0 mm. Auxiliary lighting package now available. Zinc-Alloy walker bearings replace wood blocks. Sectional flighting on lower clean grain elevator for 9600 and CTS. HarvesTrak™ now standard. Vacuum duct added to engine rotary screen. R-134A replaces R-12 in AC system. 1994 Model Changes:   90 series corn head introduced with perma-glide shields. Pressure fan added to cab air system. Beater grain curtain now available. One gallon accumulator replaces two half-gallon accumulators. Hoses replace steel lines from hydrostatic pump to motor. Straw chopper floor thickness increased to 4 mm. Straw chopper rotor changed to improve balance. Longer shoe adjusting handle. Straw walker bearing lube line added. New cab door handle. Stiffeners added to side sheets. 1995 Model Changes:   Hydraulic adjustable deck plates now option for 90 series corn head. Contour Master feeder house introduced. Factory installed only. 28L-26 rear tires and 1600cc 4-wheel drive motor now an option. Rubber belted tracks introduced. Air suspension added to operator seat. Combine data center introduced. 1996 Model Changes:   4435 combine discontinued. 1995 is the last model year. Lube interval on walker bearings increased from 50 to 200 hrs. Perforated cylinder filler plates now available. Seat belts added to all machines. Perma-Glide divider points replace steel dividers on 900 series platforms. 1997 Model Changes:   GreenStar™ offered as factory installed option. Cleaning fan high speed increased from 1350 RPM to 1600 RPM. Two new choppers available: fine cut and fine cut wide spread. Feeder house gear case oil cooler now available as an option. Crop dividers redesigned for better feeding into tine modules of CTS. Flighting pitch increased from 18 to 22 inches on 4 thru 8 row corn heads. Perma-glide deck covers changed to lower profile for better feeding. 1998 Model Changes:   Ten series introduced. Generation II - wide-spaced 13 bar concave. Enhanced cleaning system. Dual display digital tachometer. Full body side shielding. Cleaning shoe lights standard. Air flow feeder house system available. CTS II grain tank increased to 240 bushels. Variable speed feeder house drive is 20% more powerful. All combines get power increase with new PowerTech™ engines. Unloading boost increased to 33 Hp. Digital concave position display on corner post. New sun-dial type adjustment for cleaning shoe elements. New hydraulic deck plate indicator on top beam of corn heads. 1999 Model Changes:   Airflow feeder house is standard with variable speed feeder house. 800/65-32 radial becomes available. Factory installed spare knife and storage bracket installed on all cutting platforms. 2000 Model Changes:   View all new 50 series combines and the changes associated. 2001 Model Changes:   STS rice option available. Four-wheel-drive is now 2-speed. 19 row separator grates on STS corn combines. GreenStar™ Harness now in base equipment. Two fire extinguishers standard. Independent valve for reel raise and lower. 2002 Model Changes:   Corn head angle revised to level head to ground angle for improved feeding. Corn head wedge kit available for non-2002 corn heads to level head to ground angle. Timing casting changed on draper header. 19 row separator grates on all STS combines. Improved seals in cleaning shoe area for STS. Walker and CTS get grain tank design similar to STS. STS fuel capacity increased to 250 gallons. Material flow into tine module of CTS improved. Composite auger fingers introduced. Two-piece stone trap offered for field installation in very rocky areas. Threshing area components improved for reduced wear. Heavy duty grain handling option available for all STS combines. Hoses replace steel hydraulic lines on Walker and CTS. New grain handling options offered for specialty crops. AMADAS Industries attachments for specialty crops introduced. New operator seat. New gear shift lever. New Hydro handle covering. Tier II engines on 9750 STS and 9650 Walker, CTS and STS. HarvestForm™ - corn/soybean based composite introduced on panels of STS combines. 2003 Model Changes for Combines   6 cylinder, 4 valve 6.8L engine on 9550 Walker Improved feed section, top covers and concaves on STS Deep tooth chaffer and sieve is base equipment on STS 12 vane chopper tailboard and improved residue distribution GreenStar Yield Monitor included in base equipment (less display) New steering tire for 30 inch row corn (600/65R28, R1W) Dial-A-Speed and reel fore aft added to base equipment 2003 Model Changes for Heads   Dial-A-Speed and reel fore / aft standard equipment 12-ply tires standard on 36 foot draper gauge wheel 23% higher capacity draper belt drive pump 15% increase in Belt Pickup feeding capacity Hydraulic adjustable deck plates standard on 8 and 12 row corn head 2004 Model Changes for Combines   60 Series Introduction   Class 8 - 9860STS with 375 HP Class 5 - 9560STS with 265 HP 17 degree tilting front face feederhouse Single point hydraulic and electrical hookup to header equipment Electrohydraulic feederhouse reverser Higher capacity variable speed feederhouse drive standard on 9760STS and 9860STS corn combine, optional on others Improved material handling and threshing performance on STS combines Extended wear package for STS combines Increased cleaning shoe capacity for STS combines High capacity unloading system - 3.3 bu/sec on 9760STS and 9860STS TouchSet™ in cab combine adjustment for STS combines TouchSet Hydraulic system Improved moisture sensor 2004 Model Changes for Heads   600 Series Introduction   New HydraFlex cutting system Platform height sensing for rigid platforms Heavy duty knife drive 4 inch double cut knife with stone dam High capacity cross auger High capacity slip clutch 2005 Model Changes   High unload rate base on 9760STS combine High unload rate option on 9660STS combine Extra heavy duty feeder house drive base on 9660STS corn combine High capacity radiator base equipment on 9660STS and 9760STS Premium header control base equipment on all STS combines Back-up alarm base equipment on all 60 series combines Raised height skid shoes available as factory installed option Short stone dam available as factory installed option

3rd year on the 9550 here, got it at 1100 hr.   Traded a 9400 with 15 yrs use.   9550 has a lot of plusses, some enhancements to cab and controls, more power.   The 9550 will eat crop faster than an earlier machine in its class, more grain going through equals more wear on a per hour equivalent.   That may be old news to some folks but we are doing a fair amount of fixing on things which I thought should be good until 2000 hours, the previous owners kept the machine busy.  The 9550 with 1500 hrs you are looking at might need a good inspection and some questions asked to see what has been done to it.