It is the sole responsibility of the late delivering carrier to rebook and solely within CX's discretion to rebook OA.

You have not said whether you have checked luggage, but if you do, it will have been tagged to PDX on AS and all you will need to do is retrieve it for CBP and drop it post-CBP with no wait.

If you are changing carriers, you will need to have the bag retagged on UA before UA accepts it and that means waiting in another line. You now have four possible delays to deal with: Rebooking argument with CX (which may include a manager to override), CBP (you won't be at the front because you will have been dealing with rebooking), bag reclaim and security checkpoint.

Two hours is absolutely plenty for I-D in general, but add in four delays and you wind up wishing for the later flight !