"Amendment fees" refer to charges or costs that are applied when making changes or amendments to a previous agreement, such as altering a booking, modifying a contract, or revising a document. Full definition
Hello ray We wouldn't know the price difference until we amend it as you may have been given a single occupancy discount or a charge There are a standard amendment fee to pay for any changes made to bookings (loveholidays.com)
Where the Reward is a Companion Reward and the person travelling on the paying ticket changes his or her flight or class of travel, the Companion Reward may be similarly amended for the same amendment fees, subject to availability of seats and the provisions of section 4.4 of these Flying Club Terms (virginatlantic.com)
You will then need to pay a # 75 amendment fee to put new arguments (reduced to # 40 if you've managed to get the bank to agree - unlikely but you can always ask, please report successes). (moneysavingexpert.com)