Ah, that was unclear - it totally depends on what they change in the new version vs what you have changed but after many years with the cart the chances are good that you will need to redo at least some of your core code changes.
That is why it is recommended to make extensions rather than core code hacks.
There are lots of comparative tools like Beyond compare that assist in side by side comparison.
Philosophy 101 - the only thing sure in computers is there is constant change!