Want to change from html to php

newbie and I have a question I just set up my first website a month again in html. I have a dozen pages indexed on google/yahoo/alltheweb. Want to change site to php extensions. Is it to late to do that? How will the search engines react to making this change?
In other words is there a down side switching to php now? Would it be better to change the whole site to php now (leaving the index page html) instead of having to change the htaccess file later like I read in a different post. Or just leave the site in html.

what are your reasons for changing to php extentions?

You can edit your htaccess code to make your .html pages act as php but keep the .html extention

I found a program that will check my pages for link problems, spelling etc and do automatic uploads for me if the pages are in php form. Which would help me so that I'm not spending so much time upload, check, fix upload pages again etc. That way I could work on 5 or 6 pages then have the program upload them say at the end of the week or month while I'm away from the computer. On the other hand I not very good with the coding thing and trying to edit the htaccess code is a little above me.