I’m looking for a PHP/MySQL based CMS to replace my existing site.
The existing site is a magazine site built using framesets : each article is a separate HTML or PDF file, with its own metadat exposed to search robots.
The articles are not ephemeral. The aim of the site is both to provide lively and regularly updated news items, and a searchable archive which will grow over the years. There is no plan to purge articles.

I'm concerned about potential issues of backward and forwards compatibility, and would be very grateful if somebody could give me pointers to the following two issues:
1. Backwards compatibility. The site is currently based on a large number of articles, each in a separate HTML file. There should be some means of incorporating these files into the CMS without having to load them all in by hand.
2. Forwards compatibility : in the event of the site moving to different technology, it must be possible to unload the content into a standard format (HTML) such that it can easily be used to build another site. This might be provided by a backup function, or by staging the CMS into flat files (this is a standard technique in a commercial CMS, which uses the DB-driven front end for developing the site, but then exports the whole site to a flat file cache for greater speed).

Many thanks, Joel