It depends, for a reissue*, it will be calculated at the historical price anyway.

For an exchange*, it may under some circumstances be at the historical price or if the first flight coupon is touched then it is supposed to be at the fares currently in effect.

Personally I think the airlines will be extremely reluctant to exchange unflown tickets for new CAI-originating tickets. They may even argue that it is implicitly not allowed since the new ticket must be repriced at the current fare and there is no new fare to use. Alternatively, the ticket desk might quietly just use the old rate. Finally, it has been suggested that the ticket desks may try to use other fares to construct a new price, such as a series of oneway fares, but in my view this may not be possible because not all carriers issue oneway fares that can be combined in this way.

* IATA defines a reissue as a new ticket issued to replace a partially flown ticket [a change after departure]; an exchange is the new ticket issued to replace a wholly unflown ticket [a change before departure].