Looking for some advice...A client currently has a business website of static pages with htm file extensions with one hosting provider. He wants to move to php based pages as they now require dynamic content. They will also be moving to a new hosting provider but retaining their web address, moving it to the new host. For an established vertical market business they have a good ranking in Google for the exisiting pages.

The pages are being re-written in php and could retain the existing page names but with a php file extension not a htm one, or they could follow a different naming convention, that's flexible.

What is the best strategy to retain the Google ranking? Keep both sets of pages for a period on the new host with linkages between them for the googlebot to follow, start afresh with new pages and trust the googlebot visits the URL regularly anyway so will just re-index the site? what's people's views and experiences?