This won't delete the thumbnails ?
No, it doesn't, that will only happen when you also choose to purge Central Cache. But then again, purging cache only means you need to rebuild your thumbs again, takes a bit time, no more no less.
But check your prefs first (make screenshots or notes from current settings) before resetting them. They are very easy to set again to custom wishes and a reset often solves the strangest of problems for Bridge.
