Very easy to re-generate them. Select all the photos in the catalog, go up to 'Library' select 'Previews' select 'render 1:1 previews'.
If you have 10's of thousands of photos, just do it during computer down time and let it grind away.
Fresh previews directly linked to the source image with all the develop pipeline changes you've made in a place Lightroom knows how to find automatically - Seriously, I think it is far better to create new.
You will likely want to find the old previews and delete them, however, as they usually are a very large space hog.