Originally Posted by
ijgordon
I'm not exactly sure what would be causing the OPs fare to be completely re-priced, other than if there's a minimum stay requirement that isn't being met.
There's any number of possible explanations: minimum stay, Saturday night stay (these are less common than they used to be), "off-peak" travel, etc. etc. There's also special fares that have maximum stay validity (return within 30 days, return by a certain date). Theoretically, there may also be combination rules, so that (in the OP's case) maybe the new segment was only available in T and higher, but if S fares cannot be combined with other buckets, the entire itinerary has to be repriced as a T fare.