301 redirects will provide you with the best (but not guaranteed) chance of retaining SE & link juice if you NEED to move a page. If you don't need to move the page, then keep the existing URLs.
If you're converting pages to use PHP, I recommend using the methods described above (by DrTandem amongst others) to force Apache to treat pages with a .html extension as PHP files. This means you'll get the advantages of a dynamic file, but keep the current page rank with SEs and inbound links. Your URLs don't change.. everyone wins!