This will be a tough situation.

EI is an EU carrier and as such is legally required ( even though AS issued the ticket) to re-accomodate on their next flight).

Under the Montreal convention EI is also liable.
The problem is that EI, most airlines will not care and they will simply refer you back to AS who have no obligation in this case ( aside from a refund)

I would try and get AS to rebook on BA as they tend to have more award availability.

The US needs better laws protecting passengers in these situations. I don't favor compensation for cancelations ( which increases fares) but there should be laws that require airlines that require rebooking on their next flight or interline partner ( which had little impact on fares), with massive fines for non-compliance ( which will not cost airlines on penny as long as the law is followed).

Good luck!