Why downsize? They already have a fleet of ERJ135's and 145's that are flown by Express Jet.
First, I don't see us "downsizing" per se. The 735 has a high operating cost, and a 190 isn't a lot smaller than the 735. As for the 170, it would fill some of those markets where a 145 is now too small, but a 190 or a 735 has too many seats.
I think you'll see in '05 that CO
and/or XJT will order the 170, and possibly the 190.