Help! Today is the first day I can potentially book my Europe trip. The flight that I've been able to see business saver fares for EVERY DAY I have ever checked is now not showing any business saver fares on the day we need to fly out (or the day before, or the day after)

It is just before Easter, so it is possible that those fares are all gone for those days. I did find an alternative - its a non-stop between two viable airports, and economy seats are available. I was really hoping to do business on the way over, so that we could get some sleep.
If I buy the economy seats, will I have any opportunity to upgrade them later?
If I book and then want to re-book a different flight, how much will United charge me (booking on United partners).
Do I just suck it up and fly economy on the way over (like a peasant). I'm finding availability for the rest of the trip in business.
There is lots of award availability in economy (on a wide variety of carriers/mostly 1 stop), but only one non-stop flight. Do I just hold off and hope that business savers show up?
I know this is more art than science, but interested in what the hive mind has to say