How to move a website to a new host without downtime

List everything the old host does for you, not only the website files:

• Website files, including uploads and anything outside the web root that the site reads.
• Databases, with the user names and passwords the site uses to reach them.
• Email: mailboxes, forwarders and the MX records that route mail to them.
• DNS records: A and AAAA records, CNAMEs, MX, and TXT records such as SPF, DKIM, DMARC and site verifications.
• Scheduled tasks such as cron jobs, background workers and the software versions the site needs, for example the PHP version and its extensions.
• SSL certificates, redirects and rewrite rules.

Note where your DNS is hosted. If the domain uses the old host's nameservers, every record must be recreated on the new DNS before you switch.

If you keep your DNS where it is, change only the A record, and the AAAA record if you use IPv6, so that the website points to the new server's address. Resolvers pick up the change as soon as their cached copy expires, which is why lowering the TTL beforehand makes the switch fast.

Changing nameservers moves the whole DNS zone to a new provider. The new nameservers must already hold every record, and the change is published by the registry of your domain extension, where the delegation records often have a TTL of one or two days. Some visitors may still reach the old DNS during that time, so keep both zones identical until the move is complete.

Visitors should never see a certificate warning. Free Let's Encrypt certificates are usually validated over HTTP, which requires the domain to point to the new server already, so they are issued right after the DNS switch. If you need the certificate before switching, use DNS validation, which works wherever the domain points, or copy the existing certificate and its private key to the new server for the first days.

If your mailboxes move too, create them on the new server before changing the MX records, then copy the existing messages with an IMAP migration tool. Keep the old mailboxes for a few days after the switch, because some senders still deliver to the old server until their DNS caches expire. Recreate the SPF, DKIM and DMARC records for the new mail server, or outgoing mail may be rejected or marked as spam.

WordPress stores its address in the database, including inside serialized settings. When the domain stays the same, a straight copy of the files and the database is enough. If the address changes, for example from a temporary domain to the final one, replace it with a tool that understands serialized data, such as the wp search-replace command of WP-CLI, instead of editing the database by hand. Update the database details in wp-config.php, and clear every cache layer after the move: plugin caches, the web server cache and any CDN.