The full name is a friendly name. So you can have Bob or whatever you want displayed when you give access to your server or what shows up in the user switcher of your Plex home. So folks know who you are instead of having to remember who Dokuro is.
I’ll let folks know the wording of description is confusing
I’m not totally sure what you mean by “internal” accounts but managed users are not and have never been local to a server or part of your Plex account.
They are separate accounts at plex.tv. which are just linked to your Plex account as the Home Admin. They have no email or unique username. They essentially are just a friendly name (which is why they can be mostly anything) and an account number. They are linked to your account in the sense those accounts have a few special rules in that no one else can give access to their server to them but the home admin who created them
Only accounts which had their email as their username were changed. Managed accounts do not and have never had emails.
The new Full name in your profile is so you can change the visibility of your Plex account when you give access to others and what shows in the user picker. Unlike a username It does not need to be unique. You could have the full name just say “Dad” (as was asked in this feature request [Implemented] Alias instead of plex.tv usernames )
The forum software does not have the concept of a “friendly name” unfortunately that is why forum username needs to be unique. The forum will update wit your Plex account username which is one of the reason we forced the change of it being an email with @ removed. We do allow users to change the forum user name to something else (you can do so at https://forums.plex.tv/my/preferences/account ). If you change your forum username then updating your Plex account username should not change the forum username
