I've just edited the posts with the most updated info.

privatejet, that's correct. Only Y/M/U/A are upgradeable with SWUs and M/U/A are Latitude fares which are only available online. There are two types of Y, the first type is the online Freedom Y and the second type is published Y (which is what's available from the reservation agent). The online Freedom Y is about 50% cheaper than getting from a reservation agent.

Perhaps one of you can post in the Suggestions forum to request this thread to be stickied. (which is how the Best seat thread got stickied)

One major misunderstanding about fare classes:

Only international M class can be upgraded at time of booking, a pure domestic/transborder flight booked in M can only be upgraded at 7/4/2 days prior to departure depending on your status (unless it is a connection to/from an international flight).