I was under the impression that you can't do this. But while I was booking a reward ticket from YYZ-LHR where the outbound half is Business class and the return is Economy (no business class available) there was a mention on the web page of being able to upgrade. I it still cost me 90,000 pts altough the itinerary is only have Business class.

FYI - if it matters I believe at least one leg was using Priority Award and I am S100k if it matters.