Alter Username using query?

I would appreciate a little push forward...

How can I add a prefix to a Username depending on a field value?
Which plugin is best to use Auto Actions or CB Query?
I have a single select field cb_typ that can have values 1-7
If the value is 2,3,4 or 5, I want to add the value plus an underscore as prefix to the username, ex. '5_William Smith'

If it's just for display purposes I do not recommend altering the stored value. You can simply use the Profile Value Layout parameter under the Layout tab of the username field (or formatname field for usage outside of profile) to add a prefix. To add a prefix based off a field value simply use IF substitutions. See the below substitution tutorial for IF substitution usage information.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18353-using-substitutions-throughout-cb

Actually, it is not for display purposes.
I really want to alter the stored username.

We do never delete user account, we store them for statistical and historical reasons.
But users don´t have access to their accounts any longer.
Best solution had been to move obsolete account to an other DB, but I haven´t found a solution like that.

Ok, in that case you can use CB Auto Actions on the after user registration and after user update triggers. You'd need to condition for your fields then you can use a Field action to alter the username (it has a prefix operator).

Thanksm sounds promising.

I guess onAfterUpdateUser would be enough as it is all about disabling alreadt registered users.

Does onAfterUpdateUser react on both frontend and backend updates.
Does it react on CBjuice2 CSV import as well?

Thanks

I guess onAfterUpdateUser would be enough as it is all about disabling alreadt registered users.

Yes, but please also keep the below in mind as you may need this to work in both frontend and backend depending on where you're editing users.

onAfterUserUpdate = Frontend
onAfterUpdateUser = Backend

Does it react on CBjuice2 CSV import as well?

I've no idea. That'd depend entirely on if CB Juice 2 fires triggers or not.