Demo bindings never hold as well as a standard binding. I use a Din of 11 on my STH 16's and thats good for me, however if I'm testing skis even if I set the z12 to 12, I will pre release. The z12 demos are higher off the ski, dont have any adjustabe toe and they weigh a ton.
Have a look if you can get a STH 12 driver, a px 12 or a head peak 12. Any of those will work much better for you, and as has been said before, you can just get the bindings moved onto the new skis.