Massive 301 redirects & website redesign?

I've got a website that ranks position 1-3 in Google for many of my best keywords. The site is one that i've been developing since 2000 and its current state is really haphazard as I've been learning more and more as I go. I realize now that all the hacks i have in place are not the best way to design a website, and now my skills are at a point where I think I may be able to do a *major* redesign to make it look 2008 instead of a mish mash of things I've learned in bits and pieces over the years. so...

Currently all of my pages are named XXX.htm
I've implemented a .htaccess line that lets me do PHP includes (which I have a lot of) for files without a .php extension, but because of the mix of nomenclature I can't design the website in Dreamweaver and use any design mode or local viewing, which I need to do.

I want to rename all my pages to .php so that I can design the site and bring it into the modern era.

I've currently got about 6,500 inbound links that I want to preserve, can I do something on the order of 75 (yes, I have that many pages) 301 redirects and still preserve my rank and inbound traffic?

This is a major undertaking that I may or may not be able to accomplish, but I'd like to try, but I don't want to waste weeks of my time if I'm just going to somehow screw myself.

Should this work? Anything I should think about? My site is well regarded in it's niche and I'd kill myself if I screwed this up (obviously I have backups of my original files, but still...)

Thanks ahead for any info, tips, advice, etc.

OK, first of 75 pages and redirects is not a massive number so there is no problem there.

If you do a 301 redirect for all your pages you will see a drop in ranks and traffic but it will recover. You just have to be prepared for a little bit of a drop.

Why do you have to chance the URL? There really is no reason.

It could be

file.html or file.php or file

If they have the same code in they are exactly the same to a browser. As you say you have already implemented an add-handler type in .htaccess to allow you to parse php in different files.

I think you should look at the back end process to create the site and the design methods rather than messing with established well ranking URLs.

Modify your .htaccess file

AddType application/x-httpd-php .html (for .html)

AddType application/x-httpd-php .htm (for .html)

If you only plan on including the PHP on one page, it is better to setup this way:

<Files yourpage.html> AddType application/x-httpd-php .html </Files>

This code will only make the PHP executable on the yourpage.html file, and not on all of your html pages.