If Google sees your website's content change majorly, it may assume your bought the domain, right? But what if you simply redesign the page? Like I just migrated from a simple page I wrote by hand, to a wordpress cms. Will that hurt/effect page rank, or hurt/effect search results?
There is nothing wrong with buying a domain. Major changes could trigger a manual review, but as long as you have a quality site, I wouldn't worry about. It could be re-reviewed without any changes. Any changes made to your site code or content has the potential to affect rankings (positively or negatively) as it impacts the keyword density, code to content ratio (do a view source, and that is how the bot looks at your page). Page rank comes from backlinks - content changes are not going to affect your PR. Unless you are ranking in the top ten for highly competitive keywords (always be careful of making changes to what isn't broken), I wouldn't worry about a switch to wordpress. If the changes make your site a better experience for the visitor, then that is what is really important in the long end.
It might affect your ranking on some search engines like Yahoo or MSN/Live because they still care about the keyword density value, but if you have a good inbound links then no worries man... Rank well in Google and others will follow