Basically when you make your change (after departure) they are going to try to give you the same price as you would have gotten had you booked your new routing initially.

However there are some caveats, because while they will use historical pricing they will not use historical availability. So it may be that today there are extremely cheap fare buckets available but even you go to change it in 6 months there are only expensive fare buckets. They are going to give you today's prices for those expensive buckets.