Since you don't know the admin username or password the only way to find that out is to go into the backend DB and connect to the qcsiteadmin_db (whatever your corporation may have named it, what I used as a designator is the default but sometimes when doing upgrades admins will choose a different name or append it in some way, such as with the version number, to designate it as the siteadmin project DB). Once in that DB schema, you will need to expand the USERS_ROLES table and view its contents. Any USERID with the USER_ROLE_ID of 10 is a system admin account. Note these accounts with that group role ID number and then go into the USERS table and view those users. Only 1 or 2 (depending if a second was creating during installation will be the qcadmin account (and it generally shouldn't have any populated LDAP information as it is an ALM user account solely and shouldn't be on your LDAP system anywhere). Once you have located this account you can null out the password thus making the password blank. You should then be able to log in with that user account. If it doesn't work still you may need to stop and restart the services to get it to take.
Hope this helps in recovering the user account and setting this to a known password. Also make sure and annotate this somewhere available in the future as you will need this is cases where the LDAP Authentication needs to be reset to QCAUTHENTICATION and during your upgrades.
Hope this helps,