There's absolutely no reason a CMS generated site can't work for you - that's not really the SEO issue here. The HTML simply depends on you having a well written, W3C-valid template.
I would, however, not recommend using a .html file extension. If you're changing URLs anyway, go extensionless. Then your URLs are no longer dependent on the technology you use, now or in the future.
The key issue is that you are changing existing URLs. What you need to do is locate all your well linked internal pages and make sure there is a 301 redirect from the legacy URL to the new one. That will preserve most of the link juice.
You'll need to be sure that the new CMS driven site is technically nailed down - especially no canonical URL problems. I highly recommend having your CMS generate canonical link tags for every URL.
There is a chance that changing everything at once like this will give you a traffic/rankings drop for a short period, but if everything is technically sound it shouldn't last very long. You might not even feel it (some don't) but then again, you might.
There are a couple threads in the Google SEO forum that could help you: