If it is like "American Restoration", then the decals and the artwork will have to be redone. What I've done is put masking tape over decals, sure, not pretty but still better than cover them forever. With emblems, you may be able to (patiently) mold the masking tape over them.

yes your only choices are mask the labels , or reprint them.

Well I've got this case to re paint : download/file.php?id=29110&mode=view

Only the door is yellowed, however I'm pretty sure that there will be a color difference between the painted part and the original plastic, so I will have to paint everything.

I tried to paint a case once and while it didn't end terribly, it wasn't really nice. The plastic was nice (even though I sprayed a little too munch paint somewhere on it), but the metal part was getting scratched easily, like if paint wasn't really stuck to it but only white dust ...

You have to sand and prime metal before painting it. You might want to watch A LOT of painting tutorials before your try it again. I do autobody work and you essentially have a lot of prep to do before you actually spray the color onto the case even with quickie paintjobs

I searched for a few tutorials but I didn't manage to find one ... At least for PCs, and I'm not sure if tutorials made for something else would work as great on a PC (since the painting on the other case is half failed fail, I might just sand it like you told me and re-re-paint it ^^)

the tutorials would be for painting metal and plastic , although is you've never done either before I wouldn't do it to somthing you actually like. It took me years to get the proper skills to paint things perfectly.

Well I won't start with the big case I showed earlier for sure, like I said, I will just try again to paint the case I failed on. While it's not an awful case (maybe a tad cheap), it doesn't have anything special so I don't really care, I've got few other spare AT cases anyways, so I can just not use it if it goes really really wrong

The final quality of any coating (durability, etc.) will depend very heavily on how much well prepared/cleaned the surface you wanted to paint. Mechanical (sand blasting, etc..) and/or chemical (degreasing solutions, etc.) is a must for a good result.