CC - thanks! I must admit some (or much) of your response is greek to me, and I have a lot to learn about fare classes. But as you probably suspect most are my fares are simple, and booked through a TA, so the opportunity to upfare like this is presumably never (rarely?) available. And on the routes I travel (mostly domestic between mid-sized and larger cities), I don't see cheap J fares.

Would it be fair to paraphrase your situation as you booked a fairly expensive Y ticket but then figured out bumping it to J wasn't very expensive?