First, before you do anything, price out every other option and schedule. Don't worry about the last segment to SMF if it's SFO-SMF because that's a micro-hop and you should not worry about where you sit. If you can purchase that for anywhere close to what you paid AS, just have AS cancel your return (you are entitled to a full refund to your original form of payment without penalty) and purchase the new tickets.

Second, look at options as above. Train OAK-SMF is a pretty horrible option even for two adults. With an infant, forget it. But, other options which seat you in F on the Pacific segment aren't that bad, expecially if AS throws in some kind of a voucher.

Third, don't bother with the blocked seat, on that service the seat will almost certainly be needed and the best you will wind up with is trading with the middle seat guy for the aisle and that's not good. Better to see what AS can do about two adjoining aisles or even aisles which are a row apart (which people often forget about, but are pretty good with an infant).