@travel_addict: from what I've seen it's just the afternoon flights.... these were originally an ERJ at about 230pm and then a 737 at about 530pm. The morning flights appear unchanged.
OK... it's starting to get a LITTLE clearer... on expertflyer I can now that as of Sunday March 6, the two CO flights disappear and are replaced by UA CR7's.... but at least as of right now, these flights are completely zero'ed out -- you can't even buy a full fare F seat on them.
So... I think... EITHER:
1) they've just recently made the change, and haven't gotten the fares/seats published into the booking system yet... or...
2) they swapped out (ERJ+738 = about 205 seats) for (2xCR7 = about 132 seats) and they had more than 132 people already booked on the flights they changed?
Either seems possible, but the first one seems more likely... having that many seats sold more than a month in advance doesn't seem likely.