MariaDB 5 and 10

[Sneak-L8]

A year ago I installed MariaDB 10 besides version 5 and selected to migrate data to be version.

My question is, can I now de-install free MariaDB 5 ft will of be still used from QNAP firmware?

I also cannot use the hint from QNAP Q&A to update config.inc.php. As advised at some older firmware update, I added a new admin user and de-activated the built-in admin user.
Now I'm no longer able to update config.inc.php, because owner is old admin user and cannot be changed without admin's password. This seems to be changed, at least my noted password is no longer accepted.

And hints appreciated.