There is no such thing as templates in Keynote, what you would use is themes.
There are some problems with saving and applying themes in the current version of Keynote.
I am trying to save my weekly keynote presentations as templates, so that I can come back to them.
I don't see the point in doing this, and it wont work as you want it to.
Themes are used to either have a single object on every slide in a presentation or to create an over all design for a presentation that will be applied to existing slides, Apple calls this feature; themes.
if you need to alter existing presentations; make a duplicate and store this somewhere safe so you can change them to suite as necessary.