Hey Zedd

One thing that helps you rehack is if whenever you install a new hack you write down what files were changed in that hack. Then when you update your board, you look at your list of hacks, and what files they used, so you can know ahead of time what hacks will be broken by your upgrade, and what parts of the hack you have to redo.
Also, in some hacks they have comments saying that the code is part of the hack...for the hacks that don't you should put them in yourself. i.e.
#This code is for user status
That way when you use beyond compare, you know exactly what parts were added by your hacks, and what parts were added because of the upgrade.