Bumping what seems to be the most relevant thread.

I have an upcoming domestic flight that's offering a comfort+ upgrade for $79/pp (we have 3 seats). I thought I'd be able to "Upgrade with Miles" for $0.01/mile once I had enough SkyMiles in my account, so I just transfered a few 000 Amex MR points. Well, silly me, it seems that the Upgrade with Miles offer only works at the time of booking. My bad for not researching carefully enough.

On my last call with an agent, when I asked about a mileage upgrade to first class, she said there was availability in Comfort+ but not First. I didn't inquire more about Comfort+. I doubt I'll pursue the FC upgrade (i.e., waitlist), as she said it's now 30k miles one/way for an L fare (thanks for the chart DL!).

But now I still can't find any good information, is there a way to redeem a mileage upgrade award from Main Cabin into Comfort+? 10k/pp might not be a bad use for some orphan miles on a ~5 hour flight.