If someone is known for penning in a certain genre, it can be very difficult to make the switch. No, not within the writer, but with the reader.
If readers associate a writer with one type of story, it may be hard to get a fan base to follow. If they DO purchase the book, they may do so without researching the book; this may cause confusion at the least, especially going from extremes in type. For example, a horror novelist releasing a sweeping romance or a children's author switching to adult could very well be off-putting.
I don't believe an author should be forced to write one thing forever but would advise the use of a pseudonym. Sure, the ready-made audience won't be there and they probably won't all flock to an unknown novelist but it will keep things separate for readers. Besides, a writer can always reveal his/her true identity later. It would still give the new project the start it, and the author, deserve.
What would you do if you were successful in one genre and wanted to switch? What authors have handled it the best, in your opinion?