Plus as Mammamaia pointed out it makes editing your work a lot easier, you tell the punctuation with a much quicker glance. I am not sure my punctuations guides specify either way and as one is by Penguin I'll stick to my two for the time being.
I trust my word processor to correct this. Word 2007 uses one space. I didn't even know there was this debate. One sight told me to use the special backwards P symbol to show the spaces, and remove any extra spaces. (This symbol shows a dot in each space) I guess its called the paragraph show/hide key?
Word 2007 will use whatever you type in But the grammar checker gives you a choice. Go to Word Options, select "Proofing" then select "Settings..." next to "Writing style" you will see a dropdown for "Spaces between sentences", with the options "Don't check", "1" and "2". You probably don't want "don't check" -- the only position in this debate that nobody seems to be arguing for is inconsistency! Well, it shows more than just paragraph marks, so I'd call it the show/hide whitespace button.
The sight just showed the key, so I guessed at what it was called. Thanks for the assist in finding that change.
If you REPLACE the single space INCLUDING the period, I don't see why the other way around is hard work. So REPLACE [period+space] by [period+two spaces] is all you need to do. There is one caveat: If you have names like L. Pisano, you don't want two spaces between the [L.] and Pisano, and this may require manual correction.
Why actually bother with the spacing anyway? Won't it get corrected automatically by the editors if you are publishing with a big house? Alternatively, if you're self-publishing, and trying to produce a print-ready file, Word is the very last thing I'd use. I would suggest using typesetting software that will take care of correct format for you, and leave you free to worry about content.
And any other abbreviations. And ellipses. And remember that you will have to change to two spaces after question marks and exclamation marks as well...