The last part about not being able to use a tank for propane just because it was once used for NH3 falls in that category of senseless government over-reach.  If the tank is suitible for LP (and if it's suitable for NH3, it is) and it is purged and has the correct valves, there is no earthly reason it should not be able to be used for LP.  

How can they logically make this rule when the vast majority of tankers convert between both services regularly?  Changing a tank under 3000 gallons is exactly the same as changing a 9000 gallon one.