Awesome idea!
I just dropped off my old tank and new tank in order to have their necks switched... had I thought of the copper trick i would have jumped on it!
I deal with copper tubing a fair amount when at work, the copper will not degrade - gas will have no effect on the material. I might be worried about the sweat connection from the copper to steel neck on the new tank cracking over time due to road vibrations and becoming a leak point?
Again, I wish I would have thought of this.
