There is no such upgrade path which is an annoyance and something that makes me resist buying any SD movie content (which is often sub DVD quality in my view anyway).
Same goes for digital copies which are SD but seem to be becoming scarce with Blu Rays not that Ultraviolet is being pushed. I no longer buy Blu Rays as I don't want to use Ultraviolet.
Interestingly though with SD TV Shows I've found I can Complete My Season in HD discounted by the SD cost provided I've only bought a couple of episodes and the Completion price remains more than a single episode cost (if a show costs £20 for 10 episodes, or £2.49 per episode, as soon as you've spent more than £17.51 on single episodes you lose the ability to Complete and have to buy all individually- crazy!).