Swanny007 - 1:15 am on Feb 13, 2013 (gmt 0)
Ideally you don't want any URLs to change. So redirects are less desirable than keeping the URL the same. Google can handle design/template changes no problem. But change a bunch or all of your URLs and you will be hurting...
Personally I wouldn't use a CMS unless there was a strong need for it. In fact, I've taken a wordpress site and turned it into a static HTML site simply because it gave me more control over the layout and I didn't really need people commenting on the articles anyway. There are another level of issues with running a CMS: script updates, script vulnerabilities, the additional load on the server, comment spam, database backups, etc. but there are also benefits. Personally I shy away from CMSs unless absolutely necessary.
I say if you're used to doing the old HTML pages then keep doing it, but update the template, and keep the URLs the same.
If you use an app like Dreamweaver you can do a global search-and-replace for sections of the template code (stuff in the head section, etc.) that would save a bunch of time, but in reality you would have to manually check every page when you're done to make sure the process worked right. Then again, it doesn't matter how you convert the site, you should check every single page for accuracy when you're done.
Definitely the most fool-proof way to do the conversion is one page at a time. Is it possible to use SSI/PHP includes for the new template, so you can include head.html, header,html, footer.html etc. to setup the template so you can easily change it in the future?