moving a wordpress site to a new host without downtime

check email before you switch. if your MX records live at the old host and you move nameservers, mail stops arriving. copy every DNS record across, not just the A record, and send yourself a test email after the switch.

The trick is to have the new copy fully working before DNS moves. A day or two ahead, lower the TTL on your DNS records so the switch spreads quickly. Copy files and database to the new host (a migration plugin is fine for most sites), then test it by editing your hosts file so your computer loads the new server while everyone else still sees the old one. When it's all good, change the DNS. Don't make content changes or take orders on the old site during the switch or you'll lose them, and keep the old hosting live for a few days after.