Hey JoannesDecibar,
Thanks for using Apple Support Communities! If I am understanding correctly, you want to be able to delete a purchase movie from Apple TV, but ensure that you'll still be able to access that movie later one. I am happy to provide some information to help.
Purchases like movie, music, and more are connected with your Apple ID. You can safely delete those and still be able to download them without re-purchasing them, as long as you're using the same Apple ID. You can see more about downloading past purchases here: Redownload apps, music, movies, TV shows, and books
Please note, this does not included movies that are rented, only purchases.
Thanks again, and have a great day!