What I’m doing with my mailing lists is I started out by creating users for all the unique email addresses everyone has posted from over the last 20 years. Yeah, this probably created a few near-duplicate users, including me, and I know a few users who posted under multiple dissimilar email addresses over the years, as they moved around.
When I start the transition, probably next week, I will notify my subscribers that if they have posted they may already have an account under their email address, and they should try to generate a new password for it. (I assigned random 15 character passwords as I created the users.) If that doesn’t work, they have two options: Sign up for a new user account or contact me to see what email address they had posted under, in case it is an email address they can still access. (The fact that user@foo.com is, and should be, considered separate from user@bar.foo.com entered into my planning.)
I also have people who have been read-only subscribers, so they don’t have an account yet, as they’ve never posted. I may send out invitations to them to join. (I’ve already tested what happens if I try to invite someone who already has a user account under that email address, those just get bypassed.)