https://support.plex.tv/articles/201370363-move-an-install-to-another-system/
Additionally, Stop/end/quit the plex server on the source system before starting the copy of the Plex data folder.
Make the destination system the very same in regards to folder names and drive letters like the old one. Otherwise the move will be only partly successful.
If the old server has found movie A on drive letter S: in subfolder Movies/Movie A then it has to be the same on the new computer.
In preparation of future moves, you could use a dedicated hard drive just for the Plex data folder. You can then move the whole drive physically to the new machine.
If you, for whatever reason have to change drive letters and/or folder/file names, you will have to edit the plex database manually:
But even then manual poster assignments may get lost.
