LOL, as SSPhone mentioned, this is kind of a moot point since the OP hasn't returned, but in case someone does a search for "can't change from 91", I finally dug out my dusty programming manuals.

for the smaller RTCUA and RTUCB processors you program only slot 00 with the 91-94 code depending on the DTMF receiver. Slot 01 would be unused.
for the RTCUBA/RTCUBB processor pair, you still program slot 00 with 91-94, and slot 01 must be 91 (sounds like that might be the OP's case).

Apparently only the C/D and E/F pairs would take a receiver on the second card and change the slot 01 programming.