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First Flash the leaked Firmware! (you need to do a Full wipe from CWM first before you actually go trigger happy and flash via odin)

Your Modem will not be changes because the ROM itself has no modem aka Baseband, you will be left with the previously installed baseband

Rember its still not 100% confirmed if this leak has the Flash Brick Bug, SO keep every thing at your own risk dont jump it and them complain!! (So please if you brick notify every one on the thread too about it)

After Flash you will be back on Stock Recovery

You may Try Triangleaway, See to Chainfire post below!!

To Get Root and CWM5 Recovery all you have to do is Flash This CF-ROOT Package obviously by Chainfire!!
So flashing it on JB will prove no harm and you will have root and CWM5

Also After Rom Flash there is a Possibility of a boot loop if you did not do a /data and /case first befor flash of the rom

P.S just so that you know what you are doing

Many of you have a Misconception here that CF Root for SGS3 is a Kernel
Well the Answer is no its just a Repack of SuperSU Binary and the app that Roots you device as Cache.img and CWM5 Recovery that Chainfirre made for the SGS3 as recovery.img

Also when i get time i will make a Resurrection Edition one step flash that has everything

Oh and b/w i have flashed it and its working fine for me, but dont by any means think it may work for you!!


Update:

Also Official from Chain

About ROOT and CWM5:http://forum.xda-developers.com/show...6&postcount=46

About Triangle Away: http://forum.xda-developers.com/show...0&postcount=61

About the Brick Bug: #64 & #66