If i was going to build a site through regular HTML and CSS all by hand and it was indexed well by Google over a period of time would a change of mark up in example changing to a CMS set up be a problem for achieved SERP's? In this case the coding would be different but the content would stay the same, articles and other type of contextual content. I'm worried if i would ever decide to switch to some sort of CMS set up my SERP's would dramaticly plummet. Should i be worried? Or is this not really a problem? Of course the links would all stay the same, i.e: Page1.html would be Page1html in the CMS as well.
If inbound links remain the same, page urls are unchanged, internal links structure is the same and content is the same, then I can't see that changing to a CMS will affect your rankings.