Going from being 100% Straight to being 100% Gay later in life seems like a bit of a stretch to me, but turning bi seems very plausible. Perhaps it took until their 30s to find a guy they found attractive enough to consider doing something with, or maybe they've always found guys attractive but just thought it was just being able to appreciate when a guy is good looking until they got brave/curious/drunk and experimented, finding out they actually enjoyed it.
Seems more far-fetched for a gay guy to turn bi later in life as if there were any bi-sexual feelings in there at all I'm sure they would've revealed themselves when (/if) the guy was struggling with the discovery that he was gay, though it wouldn't be the first time I've tried something I didn't think I liked only to find out I did.