I don't know if this is a restriction of the changing of profile settings (until you have 20 posts) and your changes aren't saved, so essentially it would revert to the old one. Perhaps that is then reported as already taken.
I don't know what it is that you are trying to change your user name to, so I can't search and see if it is actually taken or not ?
Have you tried a more unique name -Dillon- or -Andrew- perhaps With the - either side, neither appear to have been taken. There is an andydillon though if that is what you are trying ?