There's no such thing as "was a cofounder". Once you're a cofounder, you always are, even if you're kicked out of your company or leave for any reason. Steve Jobs was still cofounder of Apple when he was kicked out of it.
:(
But I wonder if that makes you a former alumnus. Being an alumnus doesn't require a degree, does it?
Steve Case was VP of Marketing when AOL was founded as Quantum Computer Services. As his role grew, early employees left, and time passed, he became the co-founder.