Somebody with more experience or inside info will chime in but my understanding is when you change part of the ticket it is a change in the entire ticket and you come off the waitlist and my understanding was that you actually loose the upgrade money you put in but that may be only for the segment you change.
In your case B class has no fee so you did not loose the money but lost your place.
My experience in the past is that it is not fixable.
Would like to hear differently from the group.
Channa? you have experience in this situation?