I've noticed some sneaky goings on recently with CX in as much as when they are making schedule change notifications, the original booking class appears to be changing to P. You'd never know unless you actually checked. Had I not checked thoroughly, my D class ticket (more expensive than the P fare) would have flown in P (underlying fare basis remained as D) which would have put me in the same position as the OP. A call to CX resolved the issue and my ticket was correctly reissued but please be aware.