Originally Posted by
bocastephen
Use the advanced flight search function, scroll 2/3's down and enter the fare class you want to book - in this case, W.
If the systems returns with a "no flights" error, there is either no W fare on the route in question or W is sold out...just go back and select the next higher fare class above W.
Can you do this to book different fare codes for the outbound and return, though? If so--that's awesome. The OP wanted to do K and W, not W and W, which is what I think necessitated the calls. (That of course assumes that the particular K and W fares are combinable.)