If it is a TV series that you've purchased from iTunes, then it will always be available to you in your purchases tab. You can access it from the iTunes app and download it when you want to watch it. So you can delete watched episodes from your Video app and set fresh ones to download overnight ready for the next day.
Whether they be available in the cloud depends upon what country that you are in - TV shows can't be re-downloaded in all countries, and there is no guarantee that it will remain available in the store (content providers occasionally remove their items). You are better off first copying it to your computer's iTunes via File > Transfer Purchases (File > Devices > Transfer Purchases) so that you still have your own copy of the series.