I have a Specialized 2012 (or so) Roubaix Ultegra with a 10-speed cassette. Can I switch to an 11-speed cassette (Shimano Ultegra R8000) by:
1) Taking the old off and putting the new one on, quick and easy, no other adjustments, or
2) Need a conversion kit or special spacers, advanced job, or
3) Not doable. Stick to a 10 Speed.

Stick with answer 3. There's enough of a spacing difference it won't work, taking it apart which isn't possible with a number of cassettes and changing the spacers could work with some but would be more expensive than a 10 speed cassette. Plenty of cassettes use carriers for the larger gears which are stepped for each gear to individually rivet to so not really an interchangeable item. There are still a number of nicer 10sp cassettes out there that you shouldn't have a hard time finding something decent to toss on.

3(three)

Got it. Answer is 3.

This is an easy conversion to make if you buy an entire R8000 groupset instead of just the cassette. You'll also need a new freehub body on your rear wheel (could possibly require an entire new wheel) if you want to use a cassette tighter than 11-34, unless you're already running an 11-speed wheel.

Think about this: How would your 10 speed shifters function with an extra gear?


They wouldn't.

Implicit in all the excellent advice is that the bike gods are giving you a sign to buy another bike.

Convert it to a single speed and be done with it.