You would want to back up your library first using the export separate files option, (chose no to saving artwork) and then when you add sources on the new install it will read the nfo files and import the last watched status.
Another option would be to use Milhouse’s texturecache.py utility:
One of its many options is to export watched status out to a file and import it again later.
When I migrated from a standard install to a MySQL install I tried a few approaches and the one I finally used was to back up watched status using texture cache, re-scrape my library from scratch then use texture cache to re-import watched status.