When installing a new, empty, air bottle on a charged gun, is it necessary to de-gas the gun first?
I've never seen an FX tank without a valve.The "depends on the gun" comments are referring to whether or not the gun has a valved tank or not. Some newer guns have a valve in the tank that allows for the tanks to be swapped when full - a great feature for out in the field.
If your gun does not have that feature, you'll have to degas to swap the tank as there will be nothing to contain the pressure in the tank.