Sorry, I don't use jQuery.
I don't see how that code is relevant to changing the zoom level, at all.
It *LOOKS* like that code indeed is loading a *NEW* map into the "viewport".
WHY??? You only need one map. You just need to change the className of the map.
And why is a click on an <a> element used to change the zoom?? EDIT: Never mind. I see. Your + and - "buttons" are actually <a> elements. Why??? Why aren't they simply <img> elements? You can have onclick on an <img> element and then you don't have to cancel the effect of the onclick so that the <a> doesn't do the wrong thing.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.