It’s fraud, pure and simple, and serious musicians shouldn’t do it. Live performance is a contract with the audience, and part of that contract is risk on the performer’s part. The payoff, for the audience and the artist, is something unique, ineffable and transporting, all the more exciting because of the risk involved. Yes, it’s hard to sing outdoors on a cold day in front of millions of people. And sometimes it may simply be impossible to pull off, because of the weather. But don’t get involved in fraud. If you can’t do it, or it is impossible to do it, then say no. That’s part of the basic contract of live performance, too: the chance that it might be cancelled. Better to leave a hole in the program then betray your listeners.