I cannot switch accounts either. My problem is that the account that was signed into was my brother's-in-law and set up in Venezuela, but it says "YOUR REGION IS NOT SUPPORTED" and also no manner of signing out of his account so I can use it.
Paramount Plus support insists that they can help only with account issues and that all application support is through Roku.
I've tried the steps in a related article: uninstalling the app, rebooting the TV, and reinstalling it, but I get the same message. I've also check for app updates. Nothing.
Once you've signed into the app once on a Roku TV (not a separate streaming service box, it works on Xfinity's apps) the TV remembers your login for that app and there is no way to sign out of that account unless the person who signed in goes directly to Paramount Plus on a Web browser and changes their password.
Try having them do that; only then will their devices require sign in again. You may have to repeat one of the steps I did above in order to get it to work right away, however.