When deleting a version there should be an option for a "new version". All bugs which have the version to be deleted in there version fields (e.x. fixed version, target version or product version) should be updated with the given "new version".

This would make it possible to cleanup the developer versions after a release and setting all bugs to the release version for example. This would generate a overall changelog aswell.

In core/version_api.php in function "version_remove" this feature seems already to be prepared (p_new_version defaults to '' by now). So it just needs some extra question to the user when deleting versions.