FX Royale 500 with changeable air cylinder?

You are mixing words a bit - the FX 500 uses a bottle, not a cylinder. The cylinders on the Bobcats (all models) are completely interchangeable, and do not need to be degassed before removing. The bottles on the Royale seriers are also interchangeable. I have swapped bottles on my Royale - but the other way around (I put a 500 on my Royale 400). But, in that case, I do need to degass the gun. I do this by slowly loosening the fill probe. It takes around 30 minutes for me. You may be able to go faster, but I am very cautious. It is my understanding that future bottles made by FX will not need to be degassed. The Impact will use carbon fiber bottles, and those will not need be degassed.

Finally, you said "despite what FX says." Could you please site your source? Did FX tell you that on the phone; did you read it on their webpage, etc? If FX posted that information, I am sure they will want to address it re: future models.
 
just dawned on me, why would you want to change the bottle? If you are going to carry a second bottle, go ahead and get a larger 4500 psi Pony bottle, that will allow 2 or 3 or more fills in the field. Being able to fill the FX without removing the bottle makes is easy to hook up a Pony, gas up, and be on the way... With the already large shot shot, I think that you will spend much more time in the planning of getting your setup together, than you will eventually use when out in the field with your set up, though... ;-)