and I do believe that we came to the same conclusion... the weak point in a DIN regulator is how the DIN portion threads into the regulator itself. It is far thinner there, and in case of impact would break there and NOT in the tank valve. As for backing out, it just won't happen when pressure is applied. The initial reason for the difference in thread depth is for safety... they don't want you to put 200 bar regulators on a 300 bar tank. That being said, when was the last time anyone ran across a "Sport DIN" on a regulator??? It's really a non-issue today, although it is widely misunderstood.
Unless it is for some unspoken need to have all of your equipment conform or for some aesthetic reasons, I really can't see any structural reasons to not use the convertible 200 bar adapters.
BTW, 200 bar convertible valves, K valves and J valves are NOT available in the 7/8" thread (HP)... only in the 3/4" thread!!! By design, you can NOT put a 200 bar valve on a 300 bar tank. The converse is not true. The 300 bar valve IS available in a 3/4" thread.
And lest I mislead, I don't believe that UP will use or recommend ANYTHING but the 300 bar DIN. He can correct me if I am wrong in this.