I know what you mean, but I've desperately tried to change my username (thinking I could), and now I feel stuck with songalongadingdong.
There is no story behind it, and all I feel is remorse.
Personally, I think a rebrand would be fine, so long as you launch it appropriately (like, if you have an email newsletter, that would be fine).
I think people who have worked with you know you by name, and associate your name with the consistency and level of quality you have shown them.
To new people who come across your work through word of mouth or other channels, whatever name you present to them will be the one that sticks sooner or later. Even if they don't remember your name at first, if they see your quality and continue to hear good things about you, your name will stick.
Motion Rockstar has its associations.
For you to explain your story to people about why Motion Rockstar should have other associations than the ones already attached to it is a bit messy and inefficient. Even if your name has no associations at first with solid video + motion work, if you continue to make good + solid work over time, your name will eventually gain this association.
Motion Rockstar, on the other hand, is already loaded, and from the backstory you give, it sounds as if you are trying to give it new associations that don't immediately come to mind when people come across that phrase.
But, if you really love the name, stick with it.
I don't personally associate it with "douchebag" and "prick," but to me, the name is more generic than I would personally want it to be. The same goes for ninja or wizard or guru or whatever. There are many motion designers out there who have skill and sense, but only one you. Motion rockstar is very broad.