Thanks to FT, MP and some blogs, I did this very thing yesterday. We have one-way First Class tickets on Cathay Pacific in January, First Class and booked on AA. Going to LAX in May and had already booked the outbound on UA.
Called yesterday and they made JFK the stopover (for three months!) and I ended up with two First Class tickets from NYC to LAX in May! Cancelled the UA flight and deposited 50,000 miles back in my account.
This is amazing - I feel like I really just got away with something!