Changing the surname on the home person should not change your account information. If the home person is actually you, you only need to edit your surname. If it is not you, you will need to change it to you. Neither of these actions will change your account information.
I have found a problem with Ancestry missing surname matches also. It usually seems to occur where one tree has the surname occurring in more recent history than the other. As I said before, you cannot just rely on ancestry to make your matches. It takes a little more work than that. Surname match has nothing to do with your DNA actually. It is just searching both trees for names that match. If you connect the DNA to the wrong person, the search engine will be looking for the wrong names.