It should not take a full reprice. To illustrate this, I snagged a low J seat on NRT-ATL and then threw on HKG-PVG (on MU) and PVG-NRT and ATL-OMA on DL. Later found HKG-NRT on DL at low and put that on while dropping the segments over NRT. There was no low on NRT-ATL at that point, but no repricing involved. Later was able to get rid of an overnight SkyRest™ in ATL on my way back to OMA, and again no reprice.