This is SQL-Server, I assume.
For SQL-Server, on a specific SQL-Server instance used by QC, it has 2 users/passwords in SQL-Server:
db-admin (whatever name you want)
"td" this is a mandatory name assigned by QC in install.
You can easily change the "td_admin_user" password (after changing it in SQL-server) from within QC Site admin on the database server tab and it will not hurt anything for day-to-day use.
Now, not knowing the "td" user password and wanting to change it is a bit more involved, and you can test by trying to login to SQL-Server Management Studio with the user "td" and try the password you think it is.
If you don't know the "td" user password, you can change it to something you DO know.
If you migrate the databases to a NEW SQL-Server, the DB's can be used with newly created admin and "td" users on that SQL-Server, BUT it is recommended that at least the "td" SQL-Server-authentication user be created first with privs per the Install Guide (Prerequisites section regarding SQL-Server), AND after Database restore, the database must be "fixed" so it works correctly with the "td" user after restoring to the NEW SQL-Server instance by running the stored procedure to do so:
EXEC sp_change_users_login 'Update_One', 'td', 'td'
If you want to use the SAME SQL-Server instance with your NEW QC/ALM install (not recommended - it is recommended to stand up a SEPARATE SQL-Server for the NEW QC/ALM server/instance).
Please review the permissions and "td" user fixing docs on case for reference during your upgrade.
NOTE: If you try to change your "td" user password on an otherwise working ALM/QC instance, later, you may not be able to start the ALM service.
You also need to change the td user password referred to in siteadmin.xml - see this post: