In the olden days--which, I think, were only about six months ago--you could have them done a split award... XF on the outbound, XC on the return, charged for 40k for XC and 50k for XF. And, this would have been on an exception basis.

Repeatedly--and to the point that I now believe it--Reservations is saying that they will no longer make these exceptions. You must ticket at the higher award level.

DO make sure you request a waitlist of the class of service you've paid for-- your stand-by priority for that (if you don't clear in advance) will be ahead of all upgrades. You paid for the better seat, after all.