upgrading from 4.1.2 to 4.3.0 - can anyone point me to a list of MUST DO'S

I've heard that there are significant changes made between 4.1.2 and 4.3.0 so I need to know what the effects are going to be on our existing sites running old code. Is there a document that explains the effects of upgrading to 4.3.0 from 4.1.2 along with the steps I need to take to make my sites compatible with whatever PHP's new changes are? I've heard bits and pieces - which may as well be rumours - so I'd like to be able to find something in writing from the php.net site if possible. so far no luck. thanks, brian