*Knock on wood* I've weirdly lucked out on using my certificates. I burned 2 certs to JFK-CDG, 1 cert to JFK-LAX, and 1 cert LAX-JFK. I think the number of certificates is correlated with the price. So, I kept track of the fare over time, and when it went down, I cancelled one ticket (after burning 3 certs from LAX-JFK), rebooked it, and was able to get upgraded using 1 certificate.