Sorry, but nope. Its not so much about the previous contents, as it is about the pressure it was designed and tested for.
Argon tanks (which store highly pressurized gas) need to be able to withstand a much higher pressure than CO2 tanks (which store mostly liquid CO2 in a similar way to an LP tank, but at a much higher pressure than LP), and any argon mix, is also a pressurized gas.
So, you can put CO2 in an argon tank, but no argon, or mix into a CO2 tank.
As for volume of CO2:
With a molecular weight of 44, there are around 10.3mol of CO2 in a pound. Since there are 22.4L of ideal gas at STP per mole, 1 pound yields 231L, or:
8.15 Cubic Feet per pound of CO2. A heavier gas (such as propane), would yield less volume per mass (propane should be around 6.18 Cubic Feet per pound).
Some welding supliers will let You trade a tank for the one You need, Airgas is generally not helpfull in this regard, company policy. However, You might ask, just in case You find someone more lenient.