GG recommends a safe approach to switching web site IPs.
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We try to update it every few days or every few hundred fetches, I believe. I would switch IPs in this order:
- make sure that the site still works at the old IP
- bring up the site at the new IP as well
- switch the DNS to the new IP
- when you see googlebot visit the new IP to get your site, you can shut down the site at the old IP
Most of the time, you can just switch and never worry about it, but if you want to be ultra-safe, those are the steps I'd recommend.