I am writing part II of the SCEA, and I must say I find the change itinerary use-case somewhat strange.
First of all: the brief description says that both customers and travel agents may change an itinerary. This indicates that we are talking about itineraries that have already been paid for - and hance persisted to DB.
Second: nowhere in 'prepare itinerary', 'price itinerary' or 'pay for itinerary' does it say that the itinerary payment may be made later. It is described as a single workflow. Does this mean that only itineraries that have already been paid for, can be changed? How else will there be unpaid itineraries - they only exist the few minutes it takes to get from 'prepare itinerary' to 'pay for itinerary'...
Third: If only paid for itineraries may be changed, the post-conditions of the 'change itinerary' makes little sense: It states that the 'prepare itinerary' should be executed after the 'change itinerary', and that the result of the 'prepare itinerary' should be an UNPAID itinerary.
The 'new' itinerary may very well - depending on the changes made - contain flights / segments you have already paid for.