In some cases the ticket number doesn't change, for example if you just change the date, the existing ticket is 'stickered' and not reissued. The policy for travel agents is detailled here: https://www.lufthansaexperts.com/sha...ile_136168.pdf

You basically changed the reservation/PNR under a travel waiver thru the airline and the travel agent who originally issued the ticket was informed of it and required to reissue/sticker the ticket to reflect the changes.