I agree with Snuffy that Beagling for diameter is not good for best precision. But here you are with a mold that needs venting and you don't want to alter it, so you'll be running slightly oval bullets through your sizer and hoping they tend to center well enough for acceptable accuracy. If you want maximum accuracy at original intended diameter, though, you'll need to alter the mold.
As Snuffy suggested, a fly cutter run slow enough and fed fast enough so the cuts don't overlap any would put venting in, but it would have to be done carefully. I think an easier method to control for deflection is to get a jeweler's slitting saw arbor and a very thin (0.006"-0.010") thick blade
mounted and put it on a mill and cut a few horizontal slots in the face of one of the mold blocks. Deburr and away you go. I'd try about 0.005" deep to start.