Coolant in heater/AC question

[cahurst2001]

So I tore down my ventilation system today and there was probably 16 oz of coolant that came out of the vents, not the hoses. Is this a lot?

As far as I know the ventilation hasn't been changed or cleaned in 40 years. The heat/air hasn't worked since I bought the truck about a year ago. I did change the coolant and replace the thermostat shortly after I bought it.

Thoughts?

Also, how difficult is it to switch to R134a from R12?