So, how do we as writers make sure we are delivering as promised? The answer is actually easier than you might think. Communicate!
If you are switching genres, let your readers know about it. If you are trying out something new or drastically changing the tone of your books for a new project, don't hide it.
Some readers might not like this new direction. It's true. But remember that you can't please everyone. Some of your fans might not be willing to make the switch with you, and that's a risk you take. However, by not telling them, you are tricking them into reading something they might not want to. If you do this, you risk turning off your fans to all your future work. They will have lost trust in you and this can kill your fan base.
The moral of the story: Tell it like it is. If you write feel good contemporary stories then own it. Don't try to sell your work as a gritty, real life drama. If you write suspense novels with button-pushing themes, don't pretend it's a love story. Let your readers know what they're in for, and they will thank you with their loyalty.