Many (most?) airlines apply a rule that the MOST restrictive rules of an itinerary apply to ALL segments. So a booking that is "Saver/Light" (or whatever) one way and "Flexible" the other is restricted by Saver rules both ways. I have never heard of being able to change a ticket from AAA-BBB-AAA to AAA-BBB, AAA-BBB ....but that is certainly an interesting thing to try!