I found a lot of examples on how to change the username for specific revisions and so on.
But what I need is this: I did a checkout with the authentication credentia
The easiest way to do this is to simply use the --username option on your next checkout or commit. For example:
svn commit --username newUser
or
svn co --username newUser
It will then be cached and will be used as the default username for future commands.
See also: In Subversion can I be a user other than my login name?