Hi! Welcome to the forum. If you have a problem like A.S., it might be a good idea to find a highly qualified yoga therapist near you who can create a practice with reference to the specific areas your A.S. is starting. It’s important to have good range of movement exercises, but certain precautions should be taken with A.S. to prevent stress or damage, and if you want to have good results from your practice, it will be helpful to have someone who is familiar with it to give you instructions. With private yoga therapists, you may only have to meet once or twice a month, or maybe even less, and the practice is all at home in your own time.
I’ve known of a few people who do yoga that have had good results in reducing inflammation and / or maintaining healthy range of motion in their major joints. A couple of them believe that yoga has slowed the progression of A.S. and swear by it (and all of the people I’ve met with A.S. swear by therapeutic swimming too, even if they don’t do yoga!). They all had private classes with therapists or teachers, some in Iyengar style and the others with a gentle healing vinyasa style of practice (focusing on breath with movement).
All of this being said, you’d probably still be fine with a DVD or a yoga book - just keep your movements gentle and caring, and listen to your body! Yoga can be a great asset to keeping the spine flexible if you practice what is right for you. And good posture is extremely important! If you don’t know if you have it, ask your doctor or acupuncturist to advise! Proper application of yoga can also help you with posture. 
Hope this helped at all!
