Collect old addresses
Combine sitemap, internal crawl, analytics data and available search reports. For each important URL, define a new meaningful match. If the material is merged with another, the new page must actually respond to the previous request. For content removed without a replacement, return an appropriate 404 or 410 response, not a random redirect.
Continue with: Sitemap maintenance: canonical URLs and accurate dates.
Prepare signal transfer
Configure permanent server redirects to final addresses without chains. Update internal links, canonical, language alternatives and sitemap. Check images, files, forms and key metadata. In the test environment, protection against indexing is required, but it must be properly removed from public pages before launch.
Continue with: Website backups: prove that recovery works.
Check out the live site
After the release, test a selection of the most important old and new URLs, and then the full transition map. Monitor errors, search impressions and clicks. Keep access to the old structure and backup during the transition. Fluctuations in search visibility are possible; they need to be analyzed by pages, not just total traffic.
Is it possible to change CMS, domain and all content at the same time?
It is possible, but the diagnosis of the consequences is complicated. If the business allows, the changes should be divided into controlled stages.