Hello, I'm a stranger visiting from the BA forum.

There's an Avianca flight I need to be on in a few weeks time. Cash tickets are way too expensive, but I have enough United miles for an economy redemption flight. However, in a few days, I'll take a flight that will earn me enough miles to afford business class, which I'd much prefer. Here's my question: If I book the economy redemption now, can I upgrade at a later date with the difference in miles? Or am I best to wait? I'm worried about the availability vanishing in the coming week...

Thanks folks!