| Cool URLs Don't Change
I'm not sure I entirely understand your situation, but here are a few thoughts.
Cool URLs don't change - I would expect a drop in rankings (possibly for months) if you get this stuff wrong.
If, as you say, the search engines are still pointing to your .html files it may not be too late to change the extensions back and use the code I posted to have them parsed as php - but I would be paying very close attention to what GoogleBot has been doing in your site logs.
If there are only twenty files then renaming them will be a quick job, and though you will obviously have to change your internal links as well the whole process can be done very quickly with the find and replace feature of Dreamweaver or any good text editor.
External inbound links are beyond your control so if you are determined to change to the new .php extension then you need to use 301 redirects in .htaccess for all of them, though it is usually possible to do this without writing a line for each individual file.
I would not expect any difference in page loading times whichever way you go.
I wish you luck.
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