I rode my first of now three Rebels from near Clovis, NM to Cuchara Pass (9996 ft or so) almost immediately -- a 600 mile 14 hour round trip. At 9500-10,000 feet the bike maxed out at 55 indicated (high gear), but ran as smooth as normal, and started instantly -- the constant vacuum carburetor does its job very well indeed. That bike spent its entire normal life at 4700 feet, and maxed out at 83mph indicated, 78mph clocked, same as Rebel #2. Current bike has not been clocked, but seems right on the pattern. So go for it without concern.

Southwest Roadrunners - Darryl,
Blue Half Pint the Tour-Able '09 Rebel,
White Wing the Poor Man's Gold Wing '12 Suzuki Burgman 400ABS

 

You will lose performance due to the thinner air up there, but your fuel mix should go from the lean side at sea level to the rich side at altitude. I've run carbureted bikes up to about 8000 feet and they continued to run OK at these levels, so I wouldn't expect another thousand feet would make much difference.
At 9000 feet you have about 2/3 the atmospheric pressure you would have at sea level. Your Rebel might be down to 10-11 HP or slightly less due to the rich mixture. That would limit your top speed, but as long as you're patient you should get there.