Change Frequency and Priority in Sitemaps Are Ignored
Change Frequency and Priority in XML Sitemaps are not used by web search, but the lastmod date is used.
Submit Sitemaps with Changed URLs
If you change many URLs, you should submit a sitemap with the old URLs to Google which will help them pick up the redirects to the new URLs. Webmaster tools will report errors with the sitemap, because it only contains redirecting URLs, but this is fine.