Thanks to everyone for the helpful replies
I'm coming in at midday so it sounds like I'll be walking over. I have arrived on AS at SFO International but never departed from that wing (only UA) -- and to be honest I didn't realize they were separate, so now I'm looking forward to getting to explore that concourse
One last question: I assume AS will have no problem checking my bag through on BA -- but have others found AS able to print BA borading passes at the SEA lobby?