If you forgot the access credentiuals of your old plex.tv account, you need to make your server forget about them – so you can re-claim it using your new credentials (those which you used to post in this here forum).
The procedure is described in here: You need to reclaim your server, but don't dare to touch the Windows Registry
If your old plex.tv account had a lifetime Plex Pass on it, I recommend you to contact Plex Billing Support instead: https://www.plex.tv/contact/?option=plex-pass-billing
