Migration continuity
Most websites can stay online while a copy is prepared and tested. A brief controlled window may still be needed for the final data synchronisation and DNS switch.
Old site stays liveDo not cancel it while the new copy is prepared.
Dynamic data needs careOrders and edits can require a final sync.
DNS changes graduallySome visitors may reach either server temporarily.
Short answer
Static sites can normally be copied and tested without interruption. Shops, forums, booking systems and frequently updated sites need a final synchronisation so new data is not left on the old server.
Reduce risk
During propagation
DNS resolvers update at different times. Visitors may temporarily reach either server, so both should serve a usable site. Email requires the same care because cached MX records can continue directing mail to the previous service.
Keep your website online during migration covers testing, TTLs and rollback in detail.
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