I want to update a site by changing the WP theme. This, of course, may mean changing the names of categories to seam well with the theme. Will this screw up my rankings?

In theory yes! every change you make to your homepage can screw up your website rankings, but it will depend on your website SEO, type of content, keywords and keyword weight.

If you don't like the theme at all change it, "any good update, returns good results".

My advice is that once you change it work a little bit more harder in your SEO for a couple weeks and that's it.

Regards,
Me!
E-Project Business Manager
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Yes, changes done on wordpress categories and site can affect your site PR. But on the other hand it may give you more good results if you choose right keywords for your categories. Just work on off-page activities after making changes. And try to make your site user friendly.

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