At first the CO2 will simply push the guinness out of the keg and not cause any problems. Over time however more of the CO2 will disolve in the Guinness and the head will become more and more like a CO2-dispensed beer.
The flavour of the Guinness will also change considerably as this happens- dissolved CO2 has a bitter taste (think of it's effect on sparkling water such as Perrier).
I brew stouts and home and carbonate & dispense them with CO2, so if you're doing this for yourself go ahead and try CO2 if you don't mind the different head and more intense bitter flavour.
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