You have to log into the 2951 and configure it within IOS. H.323 and SIP gateways are basically independent PBXs. When the call comes in, they process it and depending on their dial plan, pass the call on to the CallManager via that H.323 trunk. So from the CallManager perspective, it's just a single device that it can send/receive calls from.
Once you log into the 2951, you can do a "show run" command and view the current PRI configs. From there, assuming the new PRIs are identical, you can duplicate the existing configs to the new ones. After that there are the dial-peers that need to also be configured. These are basically the dial-plan of the 2951, so you need to be careful here to make sure you route the calls correctly between the new and old PRIs along with the H.323 trunk to the CUCM.
If you are not familiar with IOS, I suggest talking to a network guy. They should be able to provide guidance on how to make changes to T-1 interfaces and other settings.