727 up shift flares under moderate to WOT

So, are you saying that when the throttle is WOT, then the little TP-lever on the tranny, is fully to the rear?

If yes then the next step is to check the tranny's pump pressure output. And then the oil-pressure to the servo for the second-gear band.

You may need nothing more than a KD band adjustment. If the pressure is there, then it's adjustment time. You can do this from the outside of the tranny,so, you may wish to check it first.