Hi,
you cannot backup the DB on SEP 11 and restore it on 12.1, it's technically illegal, the wizard will stop you.
I would first migrate the current manager to 12.1 and then move it to the other server with the disaster recovery procedure for 12.1 (it is not the same for 11). This is just to not waste time on installing IIS in the new server as required by SEP 11 if you want run the migration after moving the SEPM.