I think they are the same ticket class, but what the agent told me was that the availability comes from different buckets. Anyway, I also think there are plenty of examples here where you can get conflicting advice from different MPC agents. I don't know which is more accurate; I'm just reporting what I was told.

Anyway... I found the whole thing to be a real PITA. That was the first time I did an upgrade. I thought it was actually a good value. But the hoops I had to jump through to book it sucked. And then my plans changed... AND IT WAS AWFUL to unwind it. If I wanted to change, I had to do the same process of HOLDING a flight, checking with MPC on award availability (and holding the upgrade), then go back and book the ticket, and then go back to book the upgrade.

But it turned out that I just needed to cancel. For that, I had to first go to MPC to cancel the upgrade (and pay the fee). Then I had to call ticketing to cancel the flight. After enough grumbling the MPC agent said he'd handle it all. That was 10 days ago. The miles were refunded after 3-4 days. The taxes on the upgrade just got refunded. The ticket has not yet been refunded.

One final note: I think part of the hassle was I was trying to book very close to departure... like T minus 3 days. The website won't let you HOLD tickets when it's in that close. I think it would've been easier if I could have just HELD a few different reservations and then called MPC.