All future Rolemaster products will be designed for Rolemaster (RMU) but they will be usable with some degree of effort by RM2/C and RMSS/FRP GMs.
Piecemeal adoption is perhaps your best best. I suspect RM2/Classic players would happily take on the spell lists from RMU as an easy change - no gaps in lists, new high-level spells.
Best wishes,
Nicholas