Originally Posted by
IAH-OIL-TRASH
NO! NO! NO!
Unless the M-up upgrade is available when you purchase it (it will say so), you won't get a M-up post-purchase. There are M fares with space allocated for upgrades and M fares without. In your case, they did not make space available for instant M-up at the time of purchase.
That's not really an accurate description. There's no special space allocated for M upgrades. They clear into PN, as do Y/B premier upgrades and GS upgrades. An M that didn't clear at ticketing should be cleared into PN whenever PN becomes available. Additionally, the Y/B/M has priority over all CPUs and instruments within five days of departure.