How to swap a failed device

We have a failed LTM 6400 unit running 10.2.4. This device is part of a HA Pair. Yesterday we tried to replace this with a spare 6400 unit (which wasn't part of a HA pair) but ran into licensing issues. The steps we took yesterday were

• Factory reset replacement 6400 unit and plumbed in the management IP of the failed unit.
• Move this unit to the failed's rack and cabled as the failed unit
• Tried restoring using the failed's base.conf, sys.conf and then bigip.conf - However, this wouldn't allow us to authenticate using TACAS.
• to try and resolve we performed a restore from a UCS backup of the failed unit running the command - bigpipe config install (this is when we encountered the licensing error).

Are you able to advise if what we're attempting is possible and if so, what alternative steps to take to achieve this?