I would like to cut back on soda and start drinking more water but I get extreme migraines without soda. What can help me?

Sugar. That decision will benefit you a lot. The sugar content of a can of soda is equivalent to 10 tsp. If you are drinking diet, and your body can tolerate artificial sweeteners, just take it in moderation. There's some health concern before on sweeteners , e.g. Cancer causing but it was found out it is safe if taken in moderation. Colored drinks can also penetrate the enamel. Good luck!
Caffeine withdrawl? Perhaps you are getting caffeine withdrawl. Try excedrin migraine which has caffeine. Trying weaning off the soda. Drink smaller amounts each day until its done. Good luck!