As long as you know the password for your Roku account you can still log into your account. The email address is just a log in name at that point. What you cannot do is use the "forgot password" capability as it sends email only to the account on record.
Once logged in you can update your email address. When the new email address has been verified you'll use the new address to log in and it will then be the one shown on your Roku via Settings/System/About.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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