In the very near future, I will upgrade Apache and a few of the other applications on one of my servers. My question is how can I do this upgrade so that there will be minimal disruption for the search engine spiders, and so that visitors would be able to see an appropriate message about the redesign, instead of an error message from their browsers?

I know that I would need to set up an alternate server, and probably use DNS to redirect traffic to the new server, but can this be done "instantly" - change a setting when the maintenance starts, and flip it back on completion?

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