If you are going to start with a completely new library then you can export the library to individual files and that will include the watched status. You can then add into an advancedsettings.xml file the option to import watched status before scanning in your sources again and this will import your watched status when it scans the files in.
Another option would be to use Trakt to sync your watched status to the cloud.
If you wanted to skip the entire starting over from scratch thing, and you were planning on making your file structure on the new NAS the same as your existing source/s, then you could just use path substitution and redirect from the old paths to the new.