[Xenon]

a dirty way:

create a new Admin user from within your acp

then with phpmyadmin or another sql surface
[sql]UPDATE user SET userid = 1 WHERE userid = newuserid
UPDATE userfield SET userid = 1 WHERE userid = newuserid
UPDATE usertextfield SET userid = 1 WHERE userid = newuserid[/sql]

that way you got a new admin user with the userid 1.
after that, just use the build in user merge system to merge your already existing account into the new user with the id 1