This is a tough one. Overall, I'd say no, but with some caveats. First, we need to be discussing folks older than mid twenties. Earlier than that, many people are still discovering their sexuality, especially if they got a late start. Secondly, I'd be willing to give an exception for people from particularly repressive environments and such as they were probably never given the opportunity to explore that side of their sexuality. Lastly, as has been said earlier, we have to separate behavior from sexuality. For example, a repressed gay man who has only had sex with women is not straight, even if he doesn't understand that. (That being said, I am generally willing to take a person at their word unless I have lots of evidence that points in another direction.) Thus, that gay man who comes to accept his homosexuality later in life didn't change his sexuality, but rather came to terms with it.
Therefore, I'd say, as a general rule, people's sexualities don't seem to change, but seem to come to terms with it. But with every "rule" concerning human psychology (especially sexual psychology) we need to be willing to accept some exceptions. As was stated earlier, if sexuality can commonly change then we have a big problem. It leaves open the possibility that gay re-education camps could theoretically work. Secondly, the converse (or is it obverse?) could be true as well. Gay men could turn straight men gay. If so, then homophobia is not as irrational as we might think.