As with any major version upgrade you would need to get appropriate license. This is the same as for other UC systems, like CUCM or CUC, but not limited to these.
As Java wrote this is very clearly stated in the release notes for the 12.x Expressway upgrade.
You will need to reach out to Cisco to get new release keys.
Actually, Expressway 12.5.7 Release notes state: "A release key is not required to upgrade a system on X8.6.x or later software to this release (from X8.11.4 to X12.5.7 for example). This change was introduced in X12.5.4."
This is a little confusing, because it states X8.6.X+ should not require a release key, but the example it provides is from X8.11.4+
Which one is it?
Seems from OP's experience this is the latter
The release note doesn’t say “X8.11.4+” in the example, it just state X8.11.4. Nevertheless in all of the upgrades that I’ve been a part of and what the OP experienced contradict this. I’ve always had to issue a key as part of the upgrade, however I have not done any upgrades from X8.6+ to X12.5.4 or newer. The need for a new release key was changed from this release onwards. The OP did not reveal the details about the exact version that the system was upgraded to, so we wouldn’t know if this applies or not.