How much downtime should I expect when changing hosts?

RandyM asked:

I am changing hosting providers and have migrated my site to my new server space, both sites on either server are exact clones of each other. My registrar and hosting provider are different companies. I believe all I need to do to is switch the IP address for our “A” record with our registrar to the address of my new server space, keep both sites up for 48 hours or so, and can thus expect no downtime for the site as anybody visiting will see the site at one IP or the other. Is this correct? Any SEO problems?

My answer:

Aside from the rare idiots who cache DNS records for longer than the TTL given in your DNS A record (like Yandex and Baidu) you should have no issue once the old record expires.

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