@anna @adrienne @eniko @mcc all the records in your PDS contain references to your identity (DID document), it's theoretically possible to modify the data to change that but requires rewriting your entire PDS for that account, so not particularly practical.
It's the DID that is moderated against in the higher layers, not your handle.
So it doesn't matter if you're @fred.example or @jason.example, if the DID used for one becomes the DID used for another handle.
It's kinda like how on ActivityPub, software has often encoded your username into the identifier for all your posts, meaning you can't change it without breaking everything.
(Though Mastodon is starting to fix this long-standing issue, there's fix only applies on new accounts, there's no protocol level way to fix it yet โ it's kinda a weakness in JSON-LD)