Rank & conversion to PHP.....

[rj71]

Hi,
I have a large site that also includes 150 pages of html that I manually update when buyers places an Ad. I have great rankings at the moment, but to make life easier, a developer is automating my site with a database and php. These 150 pages will be renamed with a .php extension, and will be empty and dynamically created when a user opens them. How do I NOT take a rankings hit with the majority of content/Ad rich html pages on my site being of no longer importance/use?

Thanks!