Short answer
If every old URL gets a matching 301 redirect, important content is kept, and you check for 404s and submit the new sitemap right after launch, rankings usually carry over. What really causes drops is throwing old URLs away during the switch.
Our switch-over process
1) Crawl the whole old site and list every URL. 2) Give each old URL a new address and build the 301 table. 3) Test every redirect on the preview domain. 4) Lower DNS TTL before the switch and never touch mail records. 5) After launch, check redirects and 404s and submit the sitemap. 6) Keep the old site for about two weeks so we can roll back.
What you decide
You pick the switch date. We need access to your DNS account, or you move DNS to Cloudflare (our recommendation).
