I'm not so sure this is such a big of a deal. Even though members might get confused they could simply ask others or check their username history (assuming they'd go to the users profile to investigate).
You shouldn't have to feel tied down in a way though, if you feel like changing your username you should be free to do so, even though you do hold responsibilities.
How does having a picture as your profile picture imply you want to advertise it? I merely like this .Gif and wanted to make it my profile picture. It's not breaking any rules and I really don't believe anyone would find something of this sort 'offensive'.