I looked into changing a flight with Easyjet the other day. If you log into your booking and click to amend the booking you can make all the changes and get the revised price and then cancel the transaction before you confirm them. On our booking the change was £35 pp plus any difference in the flight price (we were changing the date).
I think they may have raised the amendment fees again recently, they make a heap of money on the changes people make. Makes me laugh that they call it an admin fee because we do the changes online and there is no effort on their behalf.