Those who attend and demand the oldies are just plain selfish and show a complete lack of disregard and respect IMHO. They should respect the fact that the night is advertised as a night to hear something different.
Stick to what you believe and advertise, if you lose a few punters so what, if you can live with that it won't matter. I bet they will be back before long though. It would appear that your message over the mic might have done the trick though.
Good luck with the venue.