According to this answer on How do comment @replies work? - Meta Stack Exchange
Spaces are removed from the display names for matching purposes. So to match Peter Smith you may use @pet, @peter, @peters, or @petersmith. The last two are useful if Peter Jones is also participating, who can then be distinguished using @peterj. However, no spaces are ...
It seems you cannot just change your log in, I guess one must be active at all times, I don't really know why.
You can however add more here, by clicking the add more logins button, then from the same page you can remove the previous one if you wish.
If for some reason, this still does not work, you can contact the SE team here.
You are not going to lose reputation if a user that has accepted your answer is deleted (assuming they have only accepted your answer).
After the user removal, the accept will be owned by the Community user. If you go to the Community's All actions page and select tab accepts, then you will see that Community has accepted some answers, although it hasn't ...