When you replace a trip, the price you agreed with your client stays in place. Base fare and tax come from the new PNR, Total Cost to Customer carries forward, and the Additional Amount adjusts so the numbers still add up.
If the replacement fare lands above the original customer total, Yaalago flags it and lets you edit the figures before you save, so you decide whether to hold the price or reprice the trip.
Your invoice reads the trip's accounting record, so the invoice total follows the same values with nothing to update by hand.
Exchanges work the same way. The Fully Exchange page opens with Total Cost to Customer and Additional Amount already filled in from the trip, matched per ticket, so you confirm the figures rather than enter them again.