I just tried to reroute the return leg of a full fare Y ticket with Virgin.
It was certainly possible, however they wanted in additional £280 on what was already an expensive return leg.
I assume this is because they priced the rerouted ticket at the full Y price for the new route, where as since it is sometime in the future there is still plenty of discounted fare availability on the flight I wanted.
So I decided to cancel the return leg completely and separately book a ticket on the new route... The trouble is the agent couldn't give me any clue as to how much the refund would be, and said it was unlikely to be actioned until after Christmas.