Probability? 10/10 Dead certain, since they've already spoiled it. By the look of his costume in the behind-the-scenes footage we've seen already, Pegg isn't actually going to be remotely recognisable under the various layers of foam and prosthetics. He's basically putting in a cameo that rewards himself rather than the audience, and more power to him. He's a dyed-in-the-wool Star Wars fan, getting an opportunity to act in a definitive franchise. He's living the dream. According to JJ Abrams, Pegg has actually been invaluable in terms of what he's offered in creative terms, so his on-screen presence is probably not the most important thing he's even brought to the film:
He was a sounding board. He would drop by the set because he wanted to, and when he could, he would help We could talk about things, and he was a perfect combination of incredibly smart screenwriter, fan of the series, critical fan of the series, and friend of mine but not just blindly supportive. He was an honest, critical friend. And he wasnt so close to it that he became immune to things. He would come in with fresh eyes and very smart ideas. Hes one of a handful of people along the way that has been really priceless.