Been there done that. It can't be done without cutting, welding and a bit of drilling.
The transmission is not a problem, first step is to move the inner rows to 30" spacing centered. I loosened the U bolts and with the planter in travel height I just drove the U bolts to the side with a sturdy hammer. With more muscle, it might have been easier (and for sure quicker) to have taken the row units off, then moved the U bolts and brackets and remounted the row units.
I towed it to the welding shop where I blocked up the planter, took off the wheels, and then dropped the wheel frame. My welder cut the bars for the wheels off the wheel frame, and made some new plates to weld them to, leaving the originals in case I wanted to make it wide again. It was easier to cut some new plates than to clean up the others after cutting them loose. 
Then I reassembled the planter. A flat floor was handy to align the wheel bars when welding them back. Then I slid the outer units in and moved in the drive sprocket on the drive hex shaft where I had to drill a new hole for the sprocket and ratchet assembly. Its SOP with the 4 row narrow to run the marker in the far tractor wheel track. When you can see that wheel from the tractor seat, its a far more precise driving situation than looking 30 feet ahead over the center of the tractor hood. The marker just can't be short enough for that anyway. So the marker disk is set 105" from the outside row. I also bent the marker bracket for the marker disk so the disk stood up on edge and it cut much better. I separated the marker and lift hydraulics so I could get the marker on the ground faster at the beginning of each pass. Its a good planter wide or narrow and this is how I made mine narrow. I think I paid the welder about $350 for his part, with buying a good 4 row planter for $1200 I think I did OK. I also added the fertilizer coulters, the dawn trash wheels and had the finger units gone through, plus I bought Kinze bean units. All of which increased my investment but it planted well for me. I considered getting the wheel frame from a 4 row narrow or a 6 row narrow but Kalsem Salvage said that was virtually an impossible find because of fertilizer overloading 6 row narrow wheel frames and so all in the salvage yard had gone to fix that problem. None to be had anymore Gerald J. | 
John Deere 7000 Planter
