I need to change out the AC compressor on a friends 300D. Almost all of the new or rebuilt compressors are R4 or R4/R12 compatible.

My question is if I put a newer R4 compatible compressor on what other pieces of the AC do I need to change out to convert to R4? Do I need to change out the Drier or Condenser or other parts of the AC system to be converted? or just the compressor?

Thanks.............