[j_maffe]

Yes you can. But unless you're a AAA developer studio, it's ridiculously infeasible. At least for Godot, there would be a community effort to maintain it. Such a community would be impossible with any closed-source (even if readable) engine.

[doikor]

In general one very rarely upgrade engines after release. Even during development you usually don't upgrade if there isn't a new feature in the new version you really need.

Really it is only an issue for new projects and at that point if the license of the newest version is not to your liking pick a different engine.

Also the last UE3 game I know of (Them and Us in 2021) was made by a small indie studio not some big AAA studio with massive publishers backing. At that point the engine had been in "end of life" state for 5+ years.