I assume you're referring to Personal Local Cloud? You won't have to uninstall PLC, but you will need to enter the new user's log-in credentials on the PLC client. Depending on the folders this user has access to, it could mean downloading a lot of new stuff.
I have looked all through the PLC setting and have seen no way to change which user it is configured for. The closest thing I can see is the ability to change your password in the Account settings page.
Where do I find the option to change which user the Personal Local Cloud is configured for?