Wooo. Pretty hard I would think. About the only thing the two processes have in common is that they both use a constant current arc approach. TIG uses lower voltages and higher currents while plasma uses higher voltage (100 to 160VDC). There is the whole torch thingy with the air supply, arc start mechanism, etc.
I would think it would be easier to change a Cessina 172 into a golf cart than the TIG to plasma conversion.
Watch, as soon as I say this someone will post: "I took a TIG unit and rewound the transformer and did a few mods to the torch and it made a perfect plasma unit"