Prosoundeffects.com has blog series by sound designer David Sonnenschein that covers the fundamentals of using sound effects in the sound design process. This is a great reference for those getting their feet wet with designing sound. Although the blog is focused on film sound, these concepts can be carried over to any medium.
For those who enjoyed this blog series, Sonnenschein’s book Sound Design: The Expressive Power of Music, Voice and Sound Effects in Cinema is highly recommended.
The most obvious shifts of physical space occur at the change of one scene location to another. Certainly this is motivation to change the ambient sounds to help orient the audience to the new space, but rarely is this change a dramatic turning point in itself. More likely you will have to dig for the psychological transition that can be escorted with a shift in the audio.