Once you delete them from your current system, the licenses assigned to them will revert to the pool of available licenses in your FireSIGHT Management Center.
Historical records (connections, IOCs etc.) from those devices will be retained in the original FireSIGHT.
The modules themselves will retain whatever policy you had deployed to them until you register them to the new FireSIGHT Management Center, license them from it and deploy policies from there.