Best way to upgrade?

We have PHP 4.2.1 installed on an IIS server. I want to upgrade it to 4.3.0. I downloaded 4.3.0 and am looking at the install text files and had to pose the question -- What is the best way to update a PHP install? Do I really need to install from scratch? Is there a simple way to transfer the changes I have made to the php.ini file into the new version? How can I do this and minimize the chance of creating some long-term downtime because I forgot to do something in the .ini that I (1) figured out a year ago and (2) have subsequently forgotten. Now that we use PHP for so many things, having PHP stop working because of this install would be disasterous!!!! Please advise! Thanks!