Watch out

I doubt it. Perhaps that might happen to the affected people but I think an NPC could be made to revert people to their original jobs with a click or they could just gather all the affected parties and change them back.

Except how do they know who is effected, and what job they were beforehand? If it's completely changing your class I doubt there are many remnants outside of backups, and I HIGHLY doubt Nexon would choose manually checking every single character over just rolling back everyone and giving some monetary, essentially costless compensation for the rollback. It would take a lot of time to individually change everyone, while a rollback is a quick(in comparison) server maintenance and backup load.