You need new shifters too, probably a new chain. Why not just convert the wheels to 11 speed? Often just requires a new freehub body and a re-dishing in a shop

If you want to use your old wheels just set them up 11 speed spacing with only 10 cogs and a spacer

Why do you need all your bikes to be the same number of speeds? Assuming you're going to be primarily racing the new bike then just trade in your 10 speed race wheels and buy some nice 11 speed race wheels (or convert the old ones). Assume the new bike will come with some training wheels if you don't like using race wheels all the time, if not you can pick up a cheap set.

Only issue I could see would be if you race both road and tri bikes, they're on different speeds, and you only have one set of race wheels. In which case I'd be looking at upgrading one of them to 11 speed, not downgrading the other to 10. Or just buy a second set of race wheels...

upgrading older bikes and wheels to 11s sounds like a smarter idea than mutilating a brand new bike

Or, you can trade me and I'll give you all of my Ultegra 10 spd for your 11....

This.

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No coasting in running and no crying in baseball

I didn't think of putting a spacer into the 10 to make 11.
Thanks a lot,
Harry

I went through this same exact thing (owning 3 x 10spd bikes and 2 x 11spd bikes between my wife and I). I have lots of nice 10spd wheels and was getting all flustered over it. You just need to pick up a modified cassette and call it a day! Use it on the 10spd wheels for your 11spd bike!

http://darkspeedworks.com/blog-11speed.htm

This will make it easy on you if you are like me!