To add to what LAXIntl noted, US Airways has had 4 air carrier slots for some time, and it has used CRJ-900s in those (they're too heavy for commuter slots). The fifth flight had been operated with a CRJ using a commuter slot. With Allegiant walking away, US Airways picked up one more slot, so there will now be 1 319/4 CR9s. So effectively it's more than an upgauge from a CR9 to a 319. It's really a CRJ to a 319 in terms of overall seats. By the way, it starts as an overnight at LGB
in Feb, but on Mar 4 it switches to a midday turn.
Meanwhile, Delta picked up a slot as well. It started using a 319 in LGB
last month using its previous sole air carrier slot. Now it will operate a 319 and a CR9.
JetBlue picked up a slot as well, but I haven't figured out what it's doing with it yet.