How does mod rewrite affect current search engine results?

I know most people would use mod_rewrite to help the long term effect of their search engine referrals. I'm wondering what it does to short term results?

Let's say I have something like 15,000 php pages indexed by Google but I change the url's to html pages with the page titles in the url? Google will have the old url's but when they visit the site, will be spidering the new url which aren't indexed. Also, all the inbound links will be pointing to the old page.

Will it have a strong negative short term effect?

 

301 the old URLs to the new ones and you should hardly see any negative effects. If you have tons of IBLs it's a bit hard convincing all webmasters to link to the new ones so the 301s should remain in place almost indefinitely.

 

Thank you for the answer.

 

If your site is allready indexed, why fix it when it's not broke??!!!

 

Is there a method to 301 redirect all of the pages with the old format to the new format so everything works during the transition period?

 

Options +Indexes
Options +FollowSymlinks
RewriteEngine on
RewriteBase /
RewriteRule ^whatever/(.*)$ http://www.domain.com/whatever/$1 [R=301,L]
RewriteRule ^whatever$ http://www.domain.com/whatever [R=301,L]