The TicketRevalidation message can be used to revalidate an existing ticket for a changed PNR.

When a passenger changes his flight, it may be possible to revalidate the ticket, instead of reissuing it. Revalidation is governed by IATA rulings.  IATA states that you may only revalidate a ticket to the same carrier, fare, class and routing. It is not possible to revalidate any past date segments, and you cannot change a closed flight to an open segment.

If you forget to revalidate an e-ticket, the reservation in the GDS system will not match the e-ticket data held in the airline system, and your passenger will be inconvenienced.

Important note: Not all airlines permit revalidation, in which case the ticket must be reissued with no additional collection. In addition, the validity of the ticket may determine whether it must be reissued rather than revalidated.

Note: If you get the response REVALIDATION REQUEST DENIED it is an indication that the airline does not support revalidation. Contact the airline for them to process the revalidation. Some airlines restrict the number of segments which may be revalidated at one time.