Not just the guards but I am surprised the entire population of Jerusalem didn't become believers.
Matt 27: v 53 & 54 " And the graves were opened and many of the bodies of the saints which slept arose, they came out of the tombs and went in to the holy city and appeared to many people"
So not only were the solders on guard of Jesus tomb witness to the most important miracle of all time, they would also have heard stories about the famous dead prophets of old having risen from there graves and walking around the holy city. Personally I think the event would have created quite a lot of excitement and been the social topic of the day.
The Rebel.