NathanielB - 10:28 am on Dec 3, 2012 (gmt 0)
netmeg, the only reason you would still have those old pages showing as 404s is because you didn't do your redirects!
There is nothing wrong or unsafe with changing a site to use wordpress and it shouldn't affect your rankings in the slightest as long as you keep your site structure and urls etc all the same.
The easiest way to do this (and a safer way to prevent hackers) is to install wordpress in a different folder from your root, for example install at domain.com/wp-site rather then doamin.com
In doing this, not only do it help keep your wp installation a little more hidden, but you can also work on it as a temp site without effecting your main site.
And then once your ready to go live you simple tell wordpress to display on the root domain and not the installed folder (simple to do have a google), remove your old pages (archive them in a folder or sub-domain which is blocked) and your all done :)