You can have two UR PIX's run as a failover pair, so really you don't need to do anything, just configure up failover as per normal.
If you really want to change one of the UR licenses to a FO license (no reason to though), then you need a new license key, which you can request via licensing@cisco.com, just send them a "sho ver" output from the PIX and the type of license you want.
