Thanks for the response, and sorry for being imprecise with my question!

Here's a better way of putting it. I want to book right now FRA - DXB on first class for 85,000 points and DXB - MLE on economy for 32,500. But if DXB - MLE first class availability later shows up and I want to exchange the economy ticket for the first class ticket, would I have to pay the full DXB - MLE cost (71,250 miles) or only the remaining miles for a full FRA - MLE first class ticket (so 122,500 - 85,000 = 37,500)?

Thanks again!