Great question as this is not a standard workflow. It will require leveraging some old processes with new. Basically you will first need to ensure you still have the legacy FEEDERID, FEEDERID2, and FEEDERINFO on your electric domain model. I am assuming you have confirmed this.
Once that is complete, enable FM1, disabling FM2, and run the ArcFM Trace All Feeders tool on the network in question, followed by posting the edits to Default. Here is some documentation on that step:
After the edits have been posted, disable FM1 re-enabling FM2 and Feeder Sync..
Finally, each outstanding version, upon reconciliation, will have the legacy FM1 fields populated and Feeder Sync should work going forward.
Hopefully that answers all your questions. Let me know!
Duane, a word of caution:
Take a backup of your data before doing this on production, and do this process on a test environment in order to work out any other unforeseen issues you may see.