No retune is necessary

You need the box of course
Matching rear diff ratio (3.9 ratio for 01-05 / 3.54 for 06+)
Tail shaft from same chassis Automatic model
6 speed Clutch fork
6 speed shifter linkages and gear knob
Front Stubs if its a 03+ 6 speed and matching oil seals - Assuming your stock front shafts are female
Transmission Mount

That's the bare minimum you'll need to make it happen. You can re-use all your clutch gear and starter motor as long as you use the 6 speed clutch fork. And you can re-use your transmission cross member too, but you must use the 6 speed transmission mount so that the engine and box sit level in the transmission tunnel.

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RS Liberty - Because WRX's are only good for parts...