You can stop/start your application to join the Regular or Reserves at any stage.
Up until PRMC the selection process is identical for RM & RMR & is equally applicable.
If you've initially applied for RM and passed the Recruiting Test, Medical & PJFT they remain valid. Technically you just need an update interview to switch to RMR, so a new report can be raised to confirm your suitability. The PRMC, if you've taken it is valid for 12 months however, I'd imagine the RMR would still want you to undertaqke their equivalent of PRMC- the Selection Weekend before you start RMR recruit training to affirm you have maintained your fitness levels.
If you are going for the RMR & have yet to take a PRMC, there is little to be achieved by undergoing the PRMC to be honest.