Here is the scenario:

A company releases a game client written in flash (swf) that can be downloaded and connects to the server to play their online game.
Some hacker found a way to get, for example: any item in the game (it doesn't matter what the hack is)
The company responds by releasing a "patched/updated" client (maybe even with new features) to solve the hack problem.

So, that is the scenario, but what I am wondering is the following:

How would a hacker from the new swf client, find what was changed in the swf or code and revert back to previous version, however,
with the ability to still have the new features of the client as well as ability to connect to updated server version.
so, for example: server version 1.0 (swf client 1.0)---->hack discovered on 1.0--->server version 1.1(swf client 1.1 aka "patch")
but, if 1.1 has some new features, but you only want to revert the changes done to the exploit/hack part of the client, just like
it was possible on the 1.0 client, how would this be possible to do maybe?

I was thinking of finding the differences between the two swf source codes (decompiled), but then, what would you change...
and then recompile 1.1 so that it remains with new features, however, has possibility to do 1.0 exploit on it.

I hope you guys understand. Thanks