We've had a few mergers in the last few years and have migrated SFMC accounts twice. The problem is that we now have subscriberID's with multiple SubscriberKeys. They are obviously still unique within our current instance but we have years of historical data stored on SQL servers that update based on the SubscriberID so we're now getting erroneous updates. Basically I just want to reseed our current instances SubscriberID to a number that's higher than anything from our past instances to avoid more of these collisions.
Fixing this is non trivial as subscriber keys are immutable from a users view point. The only way to fix this is to engage with your account manager and ask for a subscriber key migration. Through this process the SFMC would be able to get in through their back end ( think it’s called Synthro) and edit the tables.