That's a pretty dramatic change for only 24 hours. What struck me is that you poured the first glass on kegging, and the second (brown) glass after you started force carbing. How much trub (gunk from the bottom of fermenter) got in the keg? If you are force carbing through the liquid-out post (as many do), the co2 would have stirred all that gunk back up from the bottom into the beer, possibly causing that brown color. I wouldn't suspect oxidation that fast, but that may just be me. I would let the keg settle a couple of days, then try again. What did the 'brown' glass taste like? Oxidation has been described as a 'wet cardboard' taste. If the beer tasted very yeasty, slightly phenolic (think old bandaids), that's trub.