migrate to XHTML

The best tool to use is the HTML validator. It will look at whatever DOCTYPE you're using and tell you what needs to be changed. As you're also wanting to update a lot of files, you should also consider HTML Tidy, that can do batch across multiple files I believe: just make sure you have backups before running it over anything.

I don't think you mean xHTML 2.0 here, do you? It's only in the working draft stage right now. You probably mean xHTML 1.0.

This is from the W3c site:

This document is the eighth public Working Draft of this specification. It should in no way be considered stable, and should not be normatively referenced for any purposes whatsoever.

{{snip}}

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

You may also want to try the HTML validator extension from Firefox. It adds validation to the status bar so you can always see what the result is for a particular page and easily access a pop-up to show you the errors. That extension tends to be a bit buggy sometimes though.

This might help too:

http://cgriley.com/validator/

Won't fix anything, but if you have a sitemap it can validate the whole site at once.