I already have a website. How do I transfer it?
A transfer can usually be staged without disturbing the existing live site. If administrative/file access is available, the site can be copied to a new hosting location, updated, reviewed at a temporary location, and tested before the domain is pointed to the new version. The original site remains available during that work.
If direct administrative access is not available, a public-facing site can sometimes still be reconstructed or migrated, although it can require more work and may not include private server-side functions that cannot be accessed.