Create a new SEPM with same name and IP ( do not connec to network, it would say a duplicate name already exists)
make a note of all the keys
Install SEPM, Restore the DB & keys,
Shutdown the Existing SEPM on Weekends ( as per change management)
all the clients would failover to new SEPM. It would still be your Primary.
i'm sure there might be few who could not change over like laptops, those will come online whenever on network