It's impossible to say with 100% certainty but I'll tell you the standard response. Trademark law prevents a likelihood of confusion as to the source of a product or service. The more unique the mark, the more likely it is that people will associate a brand with one origin. For example, "Klingon Warship" - if trademarked - would only be associated with the Star Trek series. In contrast, the term "Droid" as applied to a mobile phone being possibly a Lucasfilm mark is very much stretching the boundaries of trademark law. There might be other reasons to permit this being that Verizon Wireless has no interest in challenging the claim and wants the benefit of associating Star Wars with the phone.
I'm assuming that I'm correct in associating Griffindor being some city or place in the world of Harry Potter. In your case, if you change the spelling and the words sound similar, you've already begun to create less of a likelihood of confusion with the place in the book -- assuming that it is protected. Now if you are using a name of a place (not a character) as your own stage name, there is even less likelihood that you personally would be associated as with the Harry Potter series.
I can't tell you this is the answer but it's my understanding of how trademark law might apply to your use of an alternate spelling as a surname for a stage name. Best of luck in your acting / singing / dancing career.