Plex’s data are stored in the Windows user-dependent folder
C:\Users\<username>\AppData\Local\Plex Media Server
AppData is a “hidden” folder, so you might want to activate the display of hidden files in file explorer to make things easier.
Which means that all your old library data are still there and only need to be moved/copied to the appropriate location in the AppData folder of your new Windows user account.
You must make sure that no part of Plex is running during the copy process. Otherwise the copied data may be incomplete or damaged. Use the Windows Task Manager to find running processes belonging to Plex and shut them down.
Afterwards, you must make sure that Plex is only started under the new user account. You must not have Plex running at the same time under both user accounts!
Side note: If you have a second hard drive or SSD in this machine, you should consider moving the plexdata on there instead. It only takes a small edit to the Windows registry to tell Plex to use this new location instead: [HowTo] An extended guide on how to move the Plex data folder on Windows
