I'm currently on the 757-300 on 10/1, no aircraft has been assigned yet however looking at the flight history of UA411 from ORD to LAX it looks like its common for the 738 and 739 to also be swapped in. Seat map currently is showing completely full, however I guess they could swap to a smaller a/c depending.
Currently debating on a mileage upgrade to F, I'm plat but not betting on a CPU for this trip.