It may depend on what country you live in, but in most cases you can re-download a movie for free. If the movies are already on your computer then you don't have to worry about that, and deleting from the iPad will not delete them from the computer (if they're there).
Also, over the years, I have purchased a few hundred movies in the iTunes store. I have noticed that when those movies become available for rent only in the iTunes store, they disappear from my purchases and I can not download them. Poltergeist two was one of these.