Look at one of the convertible seats like a Maxi rider. The h harness is rated up to 18 kgs, but then it becomes the booster and you can add a secondary harness to work with the lap sash belt.
Astrid, where does it state that the harness which is built in only goes up to 18kg? I can't find that... but I don't understand the reason they've done that either, other than for MORE money because you have to buy a new harness!