I have not bought a caliber kit yet, but I believe they will come with a gas block on the barrel. I would presume at this point all the barrels are shipping with the new gas block. If you have a first gen rifle, you will need to change the piston head to work with the new gas block. Rather than swap the piston head each time you change calibers (and thus gas blocks), I would order an extra new gas block to change out the one on your older barrel.
So, I believe what you want is:
-caliber conversion kit (barrel w/new style gas block, bolt, ejection port buffer, hammer spring if applicable)
-new style piston head
-a second new style gas block for your old barrel.
That is my understanding... hopefully someone will be along shortly to confirm.