I was at the same performance.
Simon Lipkin's voice went completely; he was practically William-Shatnering* the end of If You Were Gay. He'd been fine up to that point, and it was just the last few bars that went awry. He covered it well but I wasn't at all surprised to see someone else in the role afterwards.
Luke Evans did particularly well considering that he had to take over so suddenly. A few years ago I was crew on a show where the director (a professional actor) had to step in to cover one role, and he said he didn't mind whether he went on or not; the difficult bit was if he was expecting to do one thing but had to switch at short notice.
* If you've never heard William Shatner singing, I'm referring to that sort of varied-pitch speaking that people do when they haven't a hope in hell of hitting any notes. Search for "william shatner rocketman" for a (non-serious) example.