Originally Posted by
avidflyer
You can but you can just go online and do it. Go to NWA.com, Click: "More options" and then select the exact fare class you want from the pull-down.
But this probably doesn't solve the OP's problem, since one can only select the minimum fare class for both the outbound and inbound on nwa.com. Meaning, if you select "Coach Class M (or higher)" from the dropdown to get a M class on the outbound, res.nwa.com won't give you a cheaper fare class on the return, which is what the OP wants (without having to book two one-ways, I assume).