Yes if you change you page extensions, then Google and all of the other search engines would have to recrawl the entire website again and update their index. I am guessing you did not put 301 redirects in from the old URLs to the new ones?
Remember the page extensions themselves don't make a difference in the how they are ranked, but changing them without letting the search engines know greatly makes a difference in how they are found by the SE's.
Any time URLs are changed, page names, page extensions the search engines need these 301 redirects to know that the change occurred. if they don't get them you can expect to be down for a couple of months in the SERPs.
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