Apparently you can still log into your Roku account with your old password.
Will it let you change your password as a first step? Once you can log in with the new password then you can try to update your email address.
FWIW, my password is only 6 characters and I was able to change my email address. But that was several months ago, perhaps things have changed since then.
And another FYI, since I wondered about this myself, when you do get the email updated, it also changes the email address you use to log into your account and the email shown via Settings/System/About.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T