Would you make the change before or after flying the outbound portion? Wouldn't it be cheaper to just buy the restricted T ticket and then pay the change fees? That way you would at least get the T fare on the outbound part and whatever is available for the return if you make the chances after arriving at your destination. If you do the changes before travel begins, you would just be cancelling the B fare ticket, taking the refund (assuming that this is a fully refundable B fare) and purchasing the cheapest coach ticket that's available at the time you make the change.