I did a friend's SGS3 glass replacement once. Was surprisingly easy.
First off, I assume you just mean the glass, not the LCD screen underneath.
If so, I wouldn't begin to bother trying to lift the glass intact off one phone in order to use it as a replacement for another. That'd be an exercise in frustration and you're almost guaranteed to end up with two smashed screens. (Different story if you're talking about replacing the entire LCD screen from one phone to the other.)
I would just buy a replacement glass:
http://www.amazon.com/Samsung-Galaxy.../dp/B00BS3CYXY
(Word to the wise, don't take it out of the plastic until the very second you are ready to use it, wear cotton gloves or something, don't touch or get even the smallest trace of dust on the underside of the glass or you'll be stuck with it under there forever. Think: as close to clean room conditions as humanly possible if you're really doing it right.)
Remove battery, all cards.
Found this tremendously helpful: use clear packaging tape over the busted screen. The tape will hold the broken screen together rather than trying to lift up little shards of shattered glass.
Use a heat gun (or hair dryer) and a screen repair kit:
http://www.amazon.com/ProKit-Samsung.../dp/B00C1BPLAA
Heat the broken screen- heat gun works fantastic at this, just keep it moving don't melt any part of the phone- the glue under the screen will melt before anything.
Main thing I found is to use a wedge tool (looks like a guitar pick) and slowly slide it under the entire edge of the cracked screen.
The glue is sort-of like the gel type that's used to stick credit cards in envelopes that you peel off afterwards, but invisible under the screen.
Use a suction cup (get a kit that includes one) loop a finger through the ring and keep a constant upward pressure on the screen as you apply heat and wedge the edges up. (Be careful around the bottom edge buttons). Eventually, most of the screen (hopefully shards and all held together by tape) will lift right off in a single very satisfying moment. Took me about 10 minutes of this process to get to this.
Clean up the bare LCD of any stray glass particles or other debris. (I used scotch tape to lift some of the smaller more annoying little glass particles. The LCD is tougher than you think, (really like the feel of a laptop screen) but don't scrape it. I did a very careful once over on the LCD just before finishing with a camera cleaning cloth and LCD screen cleaner.
Finally, get everything like a clean-room and unpackage your pristine lens, give it the once over inspection for any sign of debris or dust on the underside- camera cloth wipe it if you need to.
Another thing that made my life easier: rather than some complicated process of sticking the touch-sensor buttons into the phone itself- merely lay your screen in 'hinge' fashion along the bottom edge and carefully stick the touch-buttons to the backside of the screen. They're actually held in place by being sticky-backed.
Then with the buttons attached (think hinge) pick the replacement screen up and lay it carefully into place- the attached buttons will help keep the bottom aligned. It should snap in place nice and satisfying-like.
Marvel at how awesome the phone already looks again.
Press the screen down, and apply more heat. I used a replacement glass that didn't have any adhesive of its own and just the leftover from the original screen re-melted and held it in place fine. (It still is). A screen with its own adhesive should be fine, but I didn't find it necessary, and frankly I don't like the idea of any debris getting stuck on the underside of it.
I let the phone 'cool' for an hour or so, and it looked brand new.
Anyway, it probably sounds like a lot of hassle but I found that it wasn't really and I'm glad I leaned how to do it without having to learn it the hard way with my own stuff.
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