Welcome to the forum.

You can check online - you don't need to keep phoning up.

If you search for a straight redemption (as if you were booking a new ticket in F) you will see if there's availability, as straight redemptions and upgrades come out of the same bucket.

Within MMB, your searches are now stored, so it's pretty easy to log in, go to "Recent Searches" and rerun it every day or so. I should know, I've been checking multiple times a day for my flight on Saturday with no joy so far!

Once you find availability you can either upgrade online or call in. But don't hang around - seats don't tend to last long!