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.

I believe the AOA website states that the bottle on the FX400 is removeable while the one on the FX500 is not. I believe it is all due to the distance from the end of the bottle to the beginning of the moderator. I have removed my FX400 bottle when shipping the gun and it did not require degassing the gun which is less than 3 years old.

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... ;-)

I have taken my bottles off on both 4 and 500s with no issues at all. On the 500 you first need to move the shroud forward or take off the barrel. They just unscrew. 
Be careful if your gun is regulated, then you will still have pressure in the action. 
Theres a bleeding screw in the bottom of the block. 

Very good Ian. Thanks for the reference. That was exactly what I wanted.

Thanks Ian. It is not cuz of the shroud is in its way on the 500. If you remove the shroud or barrel it simply screws of. Another think would be using the shorter 400 bottle.