How do I ensure I have all available quality versions of a movie?

I've found recently that for some HD movies, I either only have the 1080p versions or the 720p version on my Mac, but not both.

I think this dependes on which device I first purchased the film - if I purchase on the iPad, I get the 1080p version only... if I purchase on the iPhone, I get the 720p version only. The only time I get all three versions (1080p, 720p & SD) is when I purchase and download on the Mac.

Is there a quick way to fix this for my movies (apart from removing them from the Mac library and re-downloading them again)?