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Is there a way to switch the user license
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There is no explicit deprovisioning system for CCleaner Professional (that is more of a function for the managed endpoint products for larger businesses like CCleaner Cloud).
However, for CCleaner Professional the licensing limits are fairly robust to the kind of scenario that you describe - if you swap out a couple of old retired computers with new ones over time, you shouldn't have any trouble. Where things can go bad is if the "retired" computers start springing back into life - so if your old machines are being sold off in an intact state then best to uninstall CCleaner Professional from them before they go to someone else.
If a 1-5 seat CCleaner Professional licence seems to pop up on new computers every week (eg: it's being used as a roaming tech support tool for client computers as a one-time break-fix tool) then that is certainly going to be a problem unless you have licensed CCleaner Technicians Edition, which is specifically designed for that purpose.