Weekend Specials and SDC?
I was actually wondering this too. PMUA definitely allowed it. Not sure about PMCO. I had heard the same thing.
Was trying to see if this was somewhere within the fare rules - definitely not specifically disallowed but didn't see anything specifically allowing it on a regular ticket recently. FAQ on the same day change fee says something like it is not available on itineraries where changes aren't allowed, such as United Specials. However, the fare rules on the weekend special I saw said changes are allowed for $150. So the FAQ makes it clear as mud.