^I believe wut they said is right. The eleven speed cassette requires an eleven speed compatible freehub, and I believe the wheel is dished differently than ten speed wheels to get the cassette on the wheel with good chainline. If you have Mavic rear wheel, you might be okay since their freehub bodies are longer. And for sure Shimano changed the swingarm on the front derailer for eleven speed -- it's longer and the cable pull is most likely different than the ten speed version -- it was done to give more leverage and easier front shifting.