Can I change my sign in method (Email, Apple or Google)?
Quick answer: No. Your account stays on the method you signed up with. The only way to switch is to create a new account with the other method and cancel your existing subscription.
If you want to switch anyway
Cancel your current subscription in the App Store or Google Play (see How do I cancel my subscription? below).
Create a new account with the sign in method you want.
Your progress, sessions and favorites do not transfer to the new account.