Redoing top placement site in CMS without losing ranking

A potential client came to me today saying they really wanted to redo their existing website with a content management system and updated design. They currently manage a huge website through tons of individual HTML pages and pay someone to do the updates on an almost daily basis. They'd rather pay an upfront cost to setup a CMS and then maintain the website articles themselves.

Their biggest concern is that they are ranked #1 for many extremely competitive terms (25+ million results for these search phrases). I've already explained to him that we would redirect every old URL to the corresponding new URL and submit XML site map files (which they currently don't have). They don't even really have an SEO strategy except link trading with other websites with high PR (they have a 6/7 depending on which month you check and only trade with other 6/7 in related industries - so they've been doing a great job at that). They don't have much of a title/meta/description process.

The only reason they are still hesitating is because they feel if they lose that #1 position their business would drop drastically. Are there any reference articles anyone knows of regarding this issue of what redoing an existing, well performing site can expect to go through?

I very much like the term "if it ain't broke - don't fix it!"

To that end, why change the URLs at all? Just make the CMS system work with the already indexed and ranked pages.

You can make a custom CMS that keeps all original pages, looks exactly the same and nobody knows it has a CMS back end to produce the source.

Here is the deal

No matter how good you do with a standard CMS with redirected URLs you will lose traffic and rankings. On a competetive site it could easily experience a 6 month lull.

Keeping the existing URLs and building a CMS around the existing file structure will not lose you any rankings if you keep the content and source the same.

I would strongly advise against changing to an off the shelf CMS and changing and URLs.

Second that (or third it?)