Mandatory reading regardless of what upgrade path you choose, all for target version
A) Upgrade and migration guide
B) Release Notes
C) Compatibility matrix (depending on product might be a standalone doc, or within any of the other docs I mention)
D) README from ova
E) HW/ESXi compatibility
After reading all of the above, you'll have all the information to plan the upgrade.
If you have never done this, and specially if your environment has more than just CUCM, I STRONGLY advice a reputable consultant or a local Cisco Partner WITH voice specializations to be involved.
HTH
java
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