Migrating and upgrading

Go to the Accounts pref pane, click the lock at the lower left to authenticate, select the user you want to be an admin, and check the "Allow user to administer this computer" checkbox. You're done--that user is now an admin, and you can move over whatever you want from the newer one and delete it.

Or is there some other reason you're having a problem with a different admin user?

Admittedly, in your situation, I'd just copy everything from the imported "2" user to the other account and maintain my shortname; you'll need to change permissions on the files copied over (copying from the new account and giving admin override instead of moving will I think deal with this, incidentally), but that way you have the preferred username and all the settings you want.

Alternate plan, if you don't have much stuff in the new account: Back up anything necessary from the new account, create a temporary admin account, and delete BOTH the new one and imported one, leaving only your temporary account. NOW run the migration assistant, and you'll end up with the desired shortname and all your old stuff. Copy any backed up stuff to the newly imported user, log into it, and delete the temp account. You now have one account with all your stuff and the correct shortname.