Refer to section "Upgrading Multi-User Facility (MUF) Version 14.0 to 15.x" of CA Datacom® Core - 15.1
"...Version 15.x completely removes the outage. Version 15.x supports a CXX initialized and used as Version 14.0. In fact, 15.x can only initialize a CXX as Version 14.0. There is no such thing as a CXX set as 15.x. ..."
Refer to section "CA Datacom/AD New Features" of CA Datacom®/AD - 15.0
"...You can now perform an active Multi-User Facility (MUF) upgrade from CA Datacom/AD Version 14.0 to Version 15.0. The CXX is becoming release independent and no longer goes through a conversion process as a part of the upgrade. The MUF must be running at the CA Datacom Version 15.0 code level before performing any part of the Version 14.0 to 15.0 Active MUF upgrade process. Meaning that you simply swap the 14.0 Target Libraries with the 15.0 ones and you are there. ..."