Offering the choice to book a mixed-class journey makes sense; you could be searching til the end of time if you want to book only the same class throughout, hence, the option is offered.

If you are unable to rebook before check-in, ask to be placed on standby for an upgrade at check-in. As a PAID ($$) Z-class ticket-holder, you will be higher than eUpgrades on the standby list at the gate.

Good luck!