Greetings Bill:
I forgot to mention that there is no reason you have to withdraw assets from the 529, assuming you do not need them for alternate expenses. You can change the beneficiary and use it for expenses as an accredited education institution. You are, of course, never to old to start learning. You could also use the assets toward the education or a grandchild, niece, or nephew down the line.
I also do not wish to presume the nature of the disability, but the recently introduced 529A account - created from the ABLE Act at the end of 2014 - might be an option to consider. 529A plans are intended for individuals living with significant disabilities that have been diagnosed before they turned 26 years old, with a condition that expected to last for at least 12 consecutive months. They're still new and are likely to be modified - to the benefit of account owners - in the coming years. There are limited providers and information at this time, but these are good places to start if you're interested:
Note that the Joint Committee on Taxation’s description of the PATH Act, passed at the end of 2015, refers to a provision that would permit limited rollovers from a 529 plan to a 529 A account without penalty, but this change was not included in the final PATH Act and has not since - to my knowledge - been implemented.
I hope the aforementioned is helpful. I urge you to contact your respective 529 Plan Provider and let them know that you are interested in making a non-qualified withdrawal due to the disability of your beneficiary.They will tell you exactly what is required and guide you through the process.
Good luck,
Brian