It is not necessary to change the expansion valve for the conversion. It is recommeded, but not "necessary". You can see the valve at the firewall, I am not sure it can be separated from the evaporator w/o removing that piece. This is a job you do not want to tackle. The conversion is a compromise of sorts, so there are other components that would need replacement for max efficiency, ie. compressor, receiver/dryer and a larger condenser. Keep it simple and see what you get without swapping a lot of parts.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.