You ought to be able to get a straight refund for your BS fare - thats a perk of the fare type - and purchase your WGA fare separately. No need to worry about spending it on SW later.

Edit: too slow - thats what i get for getting distracted whilst trying to be helpful =p
Edit2: ah, unfortunately I'm unable to answer that at all, my apologies. Two things I'd consider: are you willing to let them find out that you've done this - ie, do you want the job? Even if there is technically no way they'd find out under normal circumstances, would you be willing to foot the bill if they change your interview date?