I recently booked a May trip that includes AA's codeshare from IAD to SEA on AS. AA.com put me in row 8 and didn't offer any option to change that.

Knowing from past experience and from seatguru.com that there's an extra inch of pitch in the back of Alaska's long haul 737-700s I phoned Alaska's reservations line, gave the agent who answered the phone my AA record locator, and she changed my seat from row 8 to row 19 (the first row with the extra room) with no problems. Total time involved, including navigating the phone voicemail menu, less than five minutes.