The 75S has 12 less seats than the 739. If the actual equipment is a 739, and you purchased the flight when the equipment was a 75S, there is a $75 surcharge embedded in the ticket cost for the 75S which you should be able to get refunded. Expect domestic service, but you might as well try to see if you can get access to the skyclub in SEA (you can show them the email you received if you are not granted entry with your ticket).