jw writes:

I would like to adapt the I-Valve to something more "normal" for now,
and then once the tank is empty put on a CGA-580 valve.

The big problem you can't avoid is that a fire extinguisher valve is a
dip-tube dump type, since to put out a fire you want as much directed
mass flow as possible. Connecting an ordinary gas regulator to this
liquid CO2 is asking for disaster.

Another warning is to not improvise high-pressure gas fittings unless
you can analyze and prove the strength of your work. Absolutely do not
use plumbing fittings. You do not want 75 lbs of liquid CO2 at 1000 psi
held in by a Chinese shrapnel tube designed to explode at 150 psi.

Calculate the joules represented by the containment of 75 lbs of CO2 as
warm liquid. Now divide that by, say, the 5 seconds it takes to vent a
burst high-pressure dip-tube line, to get watts, and thence to
horsepower. Humble yourself before that figure. If you don't have the
brains to make that kind of calculation, then you shouldn't be messing
with this material.

It is rather like a 750 KV power line fell into your back yard. Let's
just connect a transformer with some alligator clips to get some free
power. Whee!