Update Aug 27, 2020
The migrations have been executed. I've just tested that I can login into my old account via the new login method!
While making global login available on SEDE would be a big technical undertaking, it will still be possible for you to log in using the same email/password credential you use on the SE network, and your login flow will be mostly the same — to do this, we're creating an app on Stack Apps that will allow SEDE to authenticate SE users.
The "Log in using Stack Exchange" option will still be available in the log in page:
But where you'd previously be directed to https://openid.stackexchange.com/ and see this:
You'll now be prompted to log in to Stack Overflow instead, and then be shown a one time approval of the SEDE application on SO:
Currently the openid.stackexchange.com claims on SEDE map to:
5718SEDE users, that have SE accounts, out of which
5473accounts associated with a SO profile
245accounts without a SO profile
348SEDE users, who don’t have a SE account at all
This means that the only change for the majority of users affected will notice will be that one time approval prompt. The Q&A accounts that don't have a SO profile, will be required to join SO, and approve the application.
For the users who don't have an SE account, we will create an artificial one, so that when/if they ever log in with their old email/password credential to SEDE, they will be asked to join SO and approve the SEDE application. So for them, the process will be effectively the same as for the users in the group above.
If you wanna have a look at the PR where this is being worked on on GitHub, you can have a look here.
This should be relatively straightforward, but feel free to leave any questions or concerns below.