11,000 ft?? Hmmm..Most likely you'll be somewhere in the Sierras so you're in for a treat. Simply incredible country. You'll definitely feel the elevation the first day or two but you'll soon adjust. Just plan accordingly and don't plan on accomplishing too much the first day or so. I always used to laugh at the folks who go up planning on hiking 10-15 miles, with a couple thousand feet of elevation gain, the first day and then only make it a few miles.

As for your backpack, it all depends on what you'll be doing. Are you just doing day hikes from a basecamp? Are you going to be sticking to the trails and not going off trail? If you're actually backpacking, moving to a different location every day, is your current pack large enough? Remember, it's going to be a lot colder at 11K feet than what you're used to. I've been snowed on in the Sierras in July. That means you'll need more space for warmer clothes, etc. In any case, as long as you can fit everything into your current pack, and you're not doing any extensive off-trail hiking or mountaineering, I would suspect it should be fine.

Try to find out exactly where you'll be going. I've been all over the Sierras and might be able to give you a tip or two.