The only way to change the Roku account a Roku device is linked to is to factory reset it. That doesn't affect any other devices linked to the other account. Any subscriptions that are paid through Roku are tied to the Roku account, so if that Hulu subscription is paid through Roku you wouldn't be able to use it on a Roku device linked to a different Roku account. Subscriptions that aren't paid through Roku generally have a way to log in and out that doesn't have anything to do with which Roku account a device is linked to.
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