Trying to convert the evap coil is not recommended. None on the lcoal contractors I've talked with will do it. Too hard to flush out all the incompatible oil from so many copper tubes.
The FYI there are 2 issues with coverting from R22 to R410a. 1) Pressure. 410 runs at a higher pressure so the coil would need to be rated for both. Many made after the mid 90's are. 2) oil - the oils used to lubricate the compressors that mixed with the refrigerant is icompatible and will contminate the special oil the R410a uses. If you don't flush it all out, the compressor won't live very long.
Go with a Dry fill. You should get 8-10 years without needing to add a significant amount of refrigerant if it's installed correctly. That's long enouhg where you'll be looking for a complete systme replaecment at that time.