incrediBILL - 2:18 am on Feb 13, 2013 (gmt 0)
If you can't maintain the original paths the URL issues can be easily resolved with what's called an Apache RewriteMap where you list all the original URLs and all the new destinations in a single file, or it can be done in a PHP script, both will 301 from the old location to the new.
Also install a canonical tag in the new page with the absolute path to give Google very clear instructions that this is the new path name seems to speed up the transition process.
Of course, some people just bite the bullet and change everything at once, thinking "I'll just get the pain over with and then deal with whatever happens."
I've done it both ways, incremental changes or one big change which is like ripping of a bandaid. The advantage of incremental changes is that it becomes easier to spot the effect of smaller changes and reverse them. When you change it all at once it's like launching a new site, you can't depend on what it did before except hope those 301 redirects help keep what ranking you had intact.
Assuming the new site is all done with nice clean HTML5 and CSS3 it'll probably do well.
Whether you use CMS is negligible unless you get very heavy traffic and use a shared server which can become really problematic uder a load. I would suggest CMS with the ability to export to static HTML cache, the best of both worlds. I believe there are add-ons that do this for WordPress.
FWIW, as long as you have the ability to export to static HTML, and test to make sure it works right upfront, I personally wouldn't worry about it until the site showed it was slowing down before going down that path as I know some people running CMS live on very heavy traffic loads and it performs very nicely but it also doesn't have a bunch of crazy 3rd party add-ons installed.