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.

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Jeff Lawrence
1989 300e
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.