Yoast’s plugin is by FAR the best SEO plugin for WordPress out there. You have total control over indexation, titles, descriptions. You can even edit .htaccess within WordPress. It will generate XML sitemaps, breadcrumbs and more.
So if you’re sold on using it – but scared to upgrade, here’s the basis steps I follow.
1. Crawl the site the way it is and export the results.
2. Take a screenshot of your current plugin’s settings.
3. Install Yoast.
4. Deactivate the old plugin.
5. Activate Yoast.
6. Using the crawl and screenshot, make Yoast match the settings on your old plugin (indexation, title templates, etc).
7. Re-crawl the site. Export the results.
8. Compare the new and old crawls – make sure they look the same.
9. Decide if you want to use Yoast’s XML sitemap. If so, deactivate the old one, activate in Yoast and add it to Webmaster tools. Delete the old sitemap from Webmaster Tools.
10. Do an IMPORT of the old page/post data from the old plugin to the new (see how here: If using Platinum SEO, use this special plugin for the export/import
See my post on SEOmoz about setting up WordPress for SEO Success:
How to safely de-index tags archives: