It's a bad idea to buy the "Zero" fare because it's unchangeable under any circumstances. I suggest buying the Light fare (cheaper than Extra). You can then pay a small differential to upgrade your carry on from 10 to 15 kg in the checkout process. Also, I fly them frequently, and while they for sure will catch you if you try to board a "Zero" fare flight with a rollerboard, I also personally have never had the weight of my carry-on bag checked if I paid for it.