It's a shame that you feel that way, though keeping it only on Youtube won't keep it from getting stolen if it gets popular - there are plenty (ok, at least one. Probably, I don't really use them) of add-ons that let you download videos from Youtube, and if they want to upload it then nothing is going to stop them (I know of a youtuber who had his content stolen even though he started every video with saying his youtube username -___-).
felten
It's a shame that you feel that way, though keeping it only on Youtube won't keep it from getting stolen if it gets popular - there are plenty (ok, at least one. Probably, I don't really use them) of add-ons that let you download videos from Youtube, and if they want to upload it then nothing is going to stop them (I know of a youtuber who had his content stolen even though he started every video with saying his youtube username -___-).
But regarding your username, here's the FAQ on how to change it: http://www.newgrounds.com/wiki/faq#wiki_toc_17