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If the site is already established site with good rankings, it won't take long to recover the previous rankings assuming that the other structures and contents remain same.
The previous poster gave good advice about moving files around though. If outside sites are already pointing to a lot of your pages, then changing the file extension might mean that some links out on the web would now point to invalid pages. If the site is brand-new, you might not care as much, but it's something to bear in mind.
GoogleGuy's super-duper secret tip of the day: using square brackets is a great way to describe a search. For example, how often have you heard someone say 'Do a search for "red widgets" and look at the #3 result.' Man, if I had a nickel for every time I've heard that. :) But then did the person mean for you to search for "red widgets" in double quotes, or without the quotes?!? If you use  to delimit the search, it's much more clear. So ["red widgets"] means to use quotes, but [red widgets] means to do the search without quotes. We've been using this super-secret notation inside Google for years, and it eliminates a ton of ambiguity when you're talking to someone else. Try it, you'll like it. Plus, you'll look way smart when you get to explain it to someone else--you'll automatically look like a complete search engine expert. I'll be slightly disappointed if this notation doesn't spread like wildfire throughout the webmaster community within another 6-9 months. Square brackets when talking about searches--another radical search innovation brought to you by Google. :)
That would be another good search tip of the day: ["red widgets"¦"blue widgets" warranty] will search for documents that contain the word warranty and have either the phrase "red widgets" or "blue widgets". Using the '¦' character as an OR is pretty nice in my book. I see your point, but I'll stick with  as my search delimiters though. :)