lol dude if you try to use a hole saw on a hole thats already there (you said you wanted to make an 80mm bigger, right?), you better have some spare paint.
cos if the center guide bit doesnt have anything to bite into...which it wont, because its in the hole... the outside bit (the large one) is going to scratch the crap outta your case...and probably your fingers. if you had a SOLID piece you were working with, then a hole-saw would work great.
do you have a jigsaw? do any of your friends have a jigsaw? any of your friends dads have a jigsaw? do any of your dads friends have a jigsaw?
lol im just kiddin. but really a jigsaw with a GOOD metal-cutting blade (if your case is metal, if not get a plastic cutting blade) will be your best bet.
then you can get some edging for metal and !viola! professional hole.
"nagasama no longer drinks tecate, or streaks out of the night for anything, and he has a fine wife so he has no use for your girlfriend...she is safe..."