You can convert to C20B (acft, not convert the C20R engine), but will be some guages changed and some metal work were the compressor mates to the airframe. Call MD customer support for all the requirements, to include proper documentation.
What is the main reason you want the change? There are some advantages to the C20R including TBO, and some of the older problems can for sure be fixed now (N2 lockup). Starting difficulties can be completely eliminated with Intellistart (or whatever it is called now).