The new account name is only for you or if you contact Jagex and your old in game display name will still be the same.
Did you get an email saying your account transferred over?
You should have 2 launchers, the old Runescape one and the new Jagex one, make sure you only click the new one if saved as desktop shortcuts.
The new name should already be on the launcher when you open it, and you only need to login with the email and password you used to get on the forums.
If you try and log in with too many combos it may think someone is trying to hack you and may have blocked you for a bit?
Do not use the old log in info as it will no longer work.
Log out of the game but do not sign out of the launcher (if you get it working) and it should remember you so can skip the email verification codes. From the lobby screen click on 1 of the news posts and it should auto log you onto the forums but like I said that's after you get it working in the first place.
25-Jun-2023 23:46:27