well it only needs balanced if you touch the rotating assembly, ie the shaft or turbine/compressor,
I've never actually seen a K03 in bits, but some turbos you can unbolt the compressor and turbine housings without touching the shaft or turbine etc meaning you can swap housings without having to rebalance the shaft...
As i said though, it would only be worth messing with if the A3 turbo was ludicrusly cheap