If you're asking about replacing the failed module while the process is online, you should only replace the failed MFP with one that has the same firmware revision (I believe there were some common firmware revisions between IMMFP02 and IMMFP12). You would pull out the failed module, initialize the new one using special operation dipswitches, then make sure other dipswitch settings match the old (e.g,. address, redundancy, etc). After initializing and reinserting the new module, the configuration will copy over from the primary.
I agree -- should keep the firmware revisions the same while the process is in service. I have operated MFP12's with different firmware in the pair -- it works BUT there will be mysterious errors and intermittent 'crashes' of one or the other controller. Always reset, never took the process offline, but annoying. Best solution (if you have the $$$) was to wait for a plant outage and replace the pair with BRC400's. Works flawlessly and you get 20k more blocks to use.