Maintain SEO of a website while migrating it to WordPress?

The only way I can think of is to create a theme that contains the exact same structure as your current site.

Then you need to go to great lengths to keep WordPress form adding stuff that is intended to make your site more SEO friendly – but you said you don’t want to have your SEO disturbed – positive or negative disturbance. 🙂

So you’re going to need to be vigilant because by default there’s good stuff output by WordPress to help your site index better.