Your DOI registration authority (CrossRef, EZID?) is probably providing (or able to provide) error reports which describe broken DOIs. For example, CrossRef sends a “Resolution Report for prefix” email which describes successful and failed resolution attempts, and their service agreement requires maintaining a low failure rate.
If you have a journal which changes the URL syntax, a best practice is to re-register all DOIs for that journal to ensure the URLs are up-to-date with the registration authority.