I disagree about not having to worry about the upgrade clearing. The OP's got 4 pax on the record, so they'd all need to clear. Certainly not worth it, in my opinion, to pay a change fee and then hope that the upgrades clear on the new flights. I agree that the new C is nice, but since there's no guarantee that all pax will even be able to experience the new C if all the upgrades don't clear, it's not worth shelling out even more $$$. Plus, there's no guarantee until boarding that the new flight will ultimately be on a newly configured plane regardless of what the seat maps say since systemwide mechanical or weather issues could influence the type of plane actually flown on this route.

I agree with the OP - try to swap in FRA provided that no one gets downgraded, and if that doesn't work, enjoy the ride on the next flight.