need to replace the compressor oil, filter and expansion valve to be able to convert refrigerants.. it will work, but it will severly affect the life expectancy of your old compressor since an R12 compressor differs in specs with a 134a compressor....
i have registered my car, they don't test it... or maybe i was just lucky, besides if they do, i would just opt to have it registered "without appearance" and pay a little extra na lang.... but its just me.