How would you switch-over 1500pages static data to WP?

Hi everyone!

I want to have a discussion with ya’ll about…

“How should I move 1500pages worth of static HTML files into WP DB.”

I’m currently working for system development company in Tokyo.
There is a system that we have been using for 9years, and there are about 1500pages static html files, including system related documents/handmade blogs etc…
Those files are made way long ago before I got hired(with amateur developers), thus they aren’t SE Optimized, nor valid XHTML/CSS.

The way former developers did to SE Optimization, they used SITEMAP.XML/ROR.XML, which I don’t find anything wrong with it however there are informations about 1500 html files on both of the xmls.

So since I started learning little bit about WordPress/CMS, I suggested my boss to move everything into dynamic pages/DB so it will be MUCH better for SEO. and it’ll be much easier to edit and manage of course.

But my boss thinks that all the info on the sitemap/ror will be a waste if we DO move everything into WP, because all the static pages will be no longer existing.

Please help everyone.

How I can move 1500 static pages to WP efficiently and lovely?