are the tickets refundable if you have change of itinerary?

Yes, for most domestic routes, they can be canceled and get a partial refund (agent can only do it for you before collection or delivery). Or you can do it by yourself at railway station before the departure time as well. This is no problem. But if you mean the route for HK, it would be more difficult since they can only reture at the place you buy it. That means usually you cannot do it by yourself and very limited time for this arrangement. Thus if you are really not sure about the HK route, maybe it would be better if you buy it when you arrve there.