An A320->A319 swap is alway possible, though I wouldn't think it's likely on SFO-ORD as IME the swaps tend to put A319s on the longer flights because of the longer range of the A319. (It's fun when the GA for a transcon thinks the plane is an A320 when an A319 shows up at the gate...)

However, it could still happen.

Get row 1, check in early on line, and get to the airport early if you really want to keep your seats.