[thelist] URL Renaming, Page Rank, Strategy

[Rich Points]

Howdy,
I'm working with a client that has had many webmasters over the years.  
It seems that each one had their own unique way of naming files.  So the 
site has .htm and .html files, most of the urls have capital letters 
(something I hate), some of the urls use hyphens others use underscores 
and lots of them have spaces or %20 in them.  

here is an example url
<a 
href="/product_pages/Product_Demi_Jus%20de%20Poulet%20Lie%20Gold%20.htm">Chicken 
Demi-Glace</a>

I would like the urls to look something like this
<a href="/product_pages/demi_jus_de_poulet_lie_gold.html">Chicken 
Demi-Glace</a>

or even
<a href="/demi_jus_de_poulet_lie_gold.html">Chicken Demi-Glace</a>

This is a fairly large site that has been online for 8 years, it has a 
Google page rank of 5.  I'm afraid if I rename the files I will loose 
the page rank.  I could use .htaccess redirects to do this but I'm not 
sure if that's the way to go. 

My question is what is the best way to do this while maintain page rank? 

Thanks
Rich