Change woman's voice to man or child and make it believeable.

As a voice over artist I have a need for character voices, not monster voices. Clients want believeable voices, for dialog, narration, audio books. Being able to change male voice to female or vice versa, keeps the entire project in house, without having to work with other people and coordinate volume, mic quality, and overall reading quality. I am not referring to a simple pitch change, I already have software that can do that, but it does not provide a believable alternative voice...it is always obvious that the pitch has been changed and it sounds fake, not beliveable. Thank you.