The IMC and IR have lots in common as far as the exercises flown. The big difference is the accuracy required and the aircraft you may fly. For the IMC you are most likely to have a more basic IF equipped aircraft and for the IR they're usually the full monty with HSI/RMI and of course 2 engines.
The better equipped the aircraft the easier it will be. Previous IF experience is always a Good Thing but not strictly necessary. You should always try to fly to IR standards even now. It will help in the long run.
I don't know if you get any credit for an IMC rating. IR training is a very thorough programme. I guess you could also have a couple of goes in a simulator if you had access to one