Change user/password of a default repo

With an online installer, the repo used during installation is locked in the Maintenance tool (impossible to remove or even just change the login/pw). Is there a way to avoid that?
Say I want to use a temporary user name for installation for instance, then change it when the software is installed.

I also tested with several repos defined in config.xml. I use one for the installation, then its credentials are locked. If I disable it and use another one for updates, the first time I use it I'm asked for credentials and once they are saved, it's impossible to change them.

For 'user defined repos', this is not the case, you can use it and change the user later if needed.