Hi,
When you go to the download page for phone firmware it lists which cucm versions are compatible, example below
https://software.cisco.com/download/release.html?mdfid=281346595&flowid=46218&softwareid=282074289&release=%209.4%282%29SR1%20&relind=AVAILABLE&rellifecycle=&reltype=latest
7942/7962 SCCP IP Phone load - Compatible CUCM Versions: 5.1.3, 6.0.1, 6.1.3, 6.1.4, 7.0.2, 7.1.2 and above
cmterm-7942_7962-sccp.9-2-3.cop.sgn
You can check this based on which phone models you are upgrading.
HTH
Manish