The only possibility is to either pay the change fees for routing, or wait until D becomes available. I am facing the same situation of D availability for a N.A-> S.A sector in December for my tix!

If you really need to travel on that date, and you don't want to pay the change fee, the next option is to be downgraded to Y. This is possible without any payment.

Other than the two options above, I don't see any other possibilities.

Cheers!