If you reset your iPad can you then install any older iOS version?

I have and old iPad 2 (model MC775TY/A) with iOS 9.3.5. I want to reset it to factory settings before I give it to a relative, and I wonder if then I can update it back to iOS 9.3.5 or if it has to stay with the factory iOS version. I am not sure if the iOS 9.3.5 is going to be available for download if is considered obsolete.