Do I understand it right that you only want to mark your movies as "unseen"? In PVD just mark them all with Ctrl+A, right-click the selection, move down to the submenu "Seen" and choose "Not seen"...
You can export the items you want to reset to a text file (Title, Year), add Viewed using Excel (it's boolean, so "0", "no", "false" all likely work), and re-import. You'll can find other posts here explaining how to export and import if you're unfamiliar with those features.
I screwed up when I began using PVD and almost all my DVDs are marked as "seen". Does anyone know a way to do a mass reset of of the "seen" indicator. It will save me a lot of work. Thanks for any ideas.........