SMOC,
Apparently it is all to do with the ground radar, wiring for the lights etc... It isn't as simple as changing the yellow signs. When the tower can't see the taxiways etc and they need to light certain stop-bars for J5, J6, J10, J11 etc etc..., it needs to tie into their computer systems. So when one is renumbered, some wiring and computer stuff needs to be changed too. All too complicated for my simple mind to fully understand but there is method to the madness I am told.
That said, it does seem to be taking forever!