Hey Scott, according to their website, if you registered for the Full you can not switch to the Half, but if you still want to I would email the race director or post on their FB page to see what they say.
What I would do is stick to the original plan of running the Full. If you've put in the training time and your goal is just to finish, then see it through and just enjoy the ride, I promise you it'll be a great one. OKCMM was my first marathon a few years ago and I'll never regret it. The medal is awsome, the support is great and it's getting to be one of the biggest marathons in the country right here in our backyard! That's just my 2 cents, now go run a few miles. ;o)
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