My experience is you can change the routing as well, provided it is in the same zone. I changed the return leg BHX/CDG into LHR/CDG without problem other than one person on the phone saying that I could not do that once passed the point of return (after one initial OK by another), but initial info was confirmed by a third person I spoke with and the ticket was changed without difficulty. Fare adjustment (in that case nil) and € 50 change fee.