Not true. The valves have different threads for several safety purposes, but oxygen cylinders and argon cylinders are of the same construction.
It is true that not all cylinders are the same. C02 cylinders have a gas over liquid, and are not up to the pressures of cylinders that contain just plain compressed gas, but at the factory, oxygen, argon, helium, and nitrogen cylinders are interchangeable (before the valve is installed).
Like Allen said, a reputable filler should be able to look at the markings in the cylinder and tell you the service pressure, and if its testing is up to date.
They would probably also be able install a cga-580 valve for you (that part is unlikely to be in stock).
The good news for you, is that converting FROM oxygen to an inert gas, is easier than converting TO oxygen, because argon tanks do not absolutely need to be "oxygen clean".
The bad news, is that with the valve off, dirt can get into the tank and cause corrosion of the inside. So, your tank definately needs a good visual inspection of the inside.
Anyway, your filler should be well versed with this, or they can send it out for recertification. You're probably looking at $15 for a new hydro test (with no guarantee of passing), and another $20-30 for the valve to get this usable (plus the fill).
-- Robert