To go with the Apple TV update announced yesterday, 1080p content is coming to the iCloud movie service. Apple is also allowing people who upgrade to the new Apple TV re-download movies that they purchased on older units. Presumably, if that movie is available in 1080p they can grab the new resolution. However, it appears there's a caveat to the ability to re-download.
Some folks will have to blame HBO for the inability to re-download some of the films they previously purchased. Apple racked up some deals with major movie studios for their flicks on the iCloud movie service, but HBO blocked two major studios from participating with some content. Fox and Universal had a previous deal in place with HBO that trumped any iCloud ambitions the studios may have had.
Fox and Universal both have pre-existing deals in place with HBO. Those deals guarantee HBO exclusive windows to certain content. According to AllThingsD, that would mean if you previously purchased Rise of the Planet of the Apes, you will be able to watch again on Apple TV because HBO has a window on the flick.