New WordPress Implmentation / Old PHP conversion

Hi.

I have been using WordPress for a while now, but my current client has spent loads of time getting his site up the Google organic rankings.

Now, if we completely redo his site in WordPress (current site not in WP), he is obviously going to drop down the rankings.

His current site is in PHP.
Is there any way I could possibly re-create the entire site in WP, but somehow still retain the physical page names, e.g. sharkcagediving.php

I ask this as this is what Google has indexed and would potentially remove a lot of head aches trying to get him up the organic rankings again.