In principle it's a change of gas, wire and torch liner. In practice can be a real headache, especially if talking about fluid tight welds- first question would be what MIG, particually it's max output?
We use a Fronius twin pulse set and that does a nice job on both aluminium and stainless. I'd still always prefer to tig them when i can but on the thicker stuff there's no other option but to mig it, unless i can convince my gaffer to buy me a Dynasty 700.