As far as the harder than mig goes I am well versed in the mig process and going to tig, well it's different. Instead of just pulling the trigger while your free hand dangles at your side your tig keeps your free hand not so free.
Different because your having to dip your filler rod rythmically to get a consistent bead, or drag the filler as was explained to me(still working on it at school so by no means an expert). Then figure in that your filler is getting shorter, fingers are working the rod down to keep up. Don't touch the tungsten tip or you'll contaminate and so on.
I think the tig keeps you a bit more active than the mig especially if you throw in a foot control then I start to feel like I'm driving big trucks again except my left foot ain't doing anything. I've heard tig is easier than stick, which I'm still formulating my opinion on. Easier than mig....hmmm, I'm not thinking so but with my experience in mig and tig at this point there is more to tig. Although tig has some of the same things going as mig, it don't much care for "dirty" material, paint, rust and so on from what I've seen. When it comes to burning through stuff.....stick is still the most user friendly.
As far as converting a mig machine to tig? Don't really have anything to add about that but I don't know if it can be done. I've seen stick to tig and combo units are pretty much stick/tig combos. Buy a tig machine and it's likely to have stick capabilities.
Good luck, hopefully my limited knowledge has somehow helped.
Shawn