Good Idea are Bad Idea

My website is over ten years old. I have had this layout for many years.
Main page branches into about six pages which divide the site up into different categories. Sort of like food bing the main url then a section on meat, vegetables, deserts, snacks etc. (just an example)

domain
|
Cat1/Cat2/Cat3/etc
|||
articles, games, photogallery, etc etc content!

So from each of these category pages you branch out to the content pages with most of the text. Articles, games, etc. etc.

These main categories pages are very old and I would like to convert them from html to php because I have had to use javascript and iframes and other techniques to make them do what I want them to do. But the category pages have lots of backlinks which I would loose if I simply changed these pages over to php. (not the whole site changing just these 7 main category pages.)

So question is this: Should I make the changes to cat1.php etc but try to keep the cat1.htm/cat2.htm/cat3.htm etc and convert them over to keyword rich landing pages?

AND is this any kind of google TOS violation that they don't like. I want to remain 1000% whitehat!