Hey @Thomas_Rother, glad to see you’re trying out the ActivityPub plugin! Unfortunately it’s not currently possible to change the username (similar to how you can’t change a username on Mastodon), which is why we popped that warning there you can see in the screenshot.
I wouldn’t recommend trying to change it directly in the database, or Rails console. We will look at adding the ability to do this soon though. I definitely appreciate it’s desirable, particularly for situations like yours. We want to try to ameliorate any unforeseen issues (i.e. we’ll need to test that scenario thoroughly).