Propane Tank for CO2?

I just stumbled upon this page: How to Fill a Propane Tank With CO2 | eHow.com which suggests you could refill a propane tank with co2. Since all the valves are the same, I'm sure it can physically be done, my question is how safe would this be to use for kegging. I have several empty propane tanks of various sizes, and am considering taking them up to my local welder supply shop to have filled with CO2. My main concerns are: is this empty tank really empty, or is there still some liquid propane in it? and is this tank gonna have an odor from the old propane?

That just seems like a bad idea. Can a propane tank even handle that kind of pressure?

Propane tanks are not made to hold the same pressures that CO2 cylinders are. Propane tanks are welded tanks. CO2 cylinders are seamless tanks. CO2 tanks are built to hold pressures over 1600 PSI. Propane tanks are built to hold at a maximum of 250 psi. The normal operating pressure for a CO2 cylinder at 70°F is 800 psi.

I guess I answered my own question...

Yeah, I dont know about the same threads and size, not to mention that tank thickness. All of my c02 tanks seem pretty thick, where propane tanks seem thin. I dont think you will find any places that let you fill your own c02.