Earlier quoted context omitted.
I like this answer as usually the world changes that cannot be easily ported to code makes software shit at the end.
I like to quip that many hard technical-problems are actually three business-problems in a trenchcoat. In this case, the technical outgrowth might be heroic over-weekend crunch: Write some code that finds every affected timestamp in the DB, reconstructs the various code-chains that created them, divines an "original meaning", calculates the difference, and somehow apply changes in a way that doesn't make auditors ang…
If you have stored future times as timestamp in the database, then you also need to know with which version of the IANA timezone database those timestamps were computed. Fun ensues. Or one could use the correct data time format for the application use case. It depends on how much you value your sleep and weekends.