ZippyRedPlumber

I would say no, although some hacks are rather extensive and can blur this line a bit.

normal hacks, no...

but a hack where the game engine itself is all that's left or even part of the engine is modified as well as the look(graphics)/goals/maps/levels/types of challenges, making the game something else... that's a homebrew...

simply putting one games graphics into another and dressing it up is not a homebrew, nor is simply changing the sprites, or just modifying the code to fix something or correct a perceived failing.

does this help nail it down for you a little bit?

Nope. Not without the addition of a fair bit of new code they don't. If you change around some sprites and colors that is just a hack.