I'm not certain you can do it that way. I believe you have to remove the Roku from the current account, then reset the Roku and attach it to the new account during setup. You're going to lose all your channels when it disconnects from the other account anyway, so doing the System Reset is a clean way to reset everything. Since the channels installed on the Roku are tied to a user account, not the device itself, you can't keep your current channel settings when moving to a new account.
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