I'm a little unclear on how the upgrade certificates work. Am I right that I have to 'purchase' one for a specific flight at a specific time? In other words, let's say I have 3100 points, and I want to upgrade JFK-LAX, so I redeem 650 points for the certificate. So now I have 2450 points. Then assume the upgrade doesn't clear. Now, I am left with 2450 points and a BXP1 certificate?

In other words, I can't 'refund' the certificate, then use the 3100 miles on a BXP2 upgrade JFK-LHR, right?

By comparison, if I wanted to upgrade JFK-LAX using stickers, I would need 5 stickers, I could request an upgrade, but if the upgrade doesn't clear, the stickers are unused, and I can use a few more stickers to upgrade JFK-ZRH or whatever. I think the BE upgrade system is less flexible but it wasn't totally clear to me.

I've only earned points, not redeemed any yet, but trying to learn the ins and outs before I do - thanks in advance for any tips.