MEI1 redetermines all pricing condition types that have undergone a change. Given that your freight vendor is populated through a condition record, MEI1 should show this is the PO document worklist to for pricing updation.
Run this report using 'New price determ. for changed price elements, adopt manual' in type of automatic change and see if the relevant POs are displayed in the worklist. If only your vendor has changed in the pricing condition, the same will get updated. Also, any manual conditions you had earlier added will be adopted from the previous pricing