Do you have a VCS? If so you could use the VCS to interwork the call from Sip to H323.
Assuming you have a VCS, you can create a route pattern in CUCM to route H323 calls to your VCS. From the VCS you can have interworking enabled so that the signaling can be changed to H323 and then connect to your old VTC systems.
If you have to IP dial the old VTC systems and they do not have an e164 or H323id address, you can accomplish this using the same method with the VCS. See this guide on page 47. http://www.cisco.com/c/dam/en/us/td/docs/telepresence/infrastructure/vcs/config_guide/Cisco_VCS_Cisco_Unified_Communications_Manager_Deployment_Guide_CUCM_8_9_and_X7-2.pdf