I came across this customer where we upgraded their CUCM from 6.x to 7.x and changed the mother board of the UCS Server (there were some issues with it).
After the upgrade the Call manager had major Licensing issues as the MAC address got changed. All the units went into negative and there was an error in red saying Call manager is running out of license blah blah..
There is a process of re-homing your license if the MAC address has been changed. One way could be to send any of the previous license files to Cisco licensing email@example.com asking them to rehome it to new MAC address. You can also send an email to the same address with something on these lines:
We have license tied to PAK code YYYYYYY for our CUCM ver 7.1.3. The hardware on the Call manager was replaced and we would like the license to be re-homed to our new MAC address 45EF.AB43.BC78. The Enduser name is Company Ltd with blah blah address.
Cisco Licensing team has been very helpful in sorting out these kind of issues.