No. You'll have Confluence offline anyway when you shut it down to change the database server - you can't do that while it's running.
More generally, there's nothing in Confluence that would lock out the users. It's either active and available, or it's not.
Externally, I've needed to do things like this many times, and I usually have Confluence behind a proxy. With a proxy, it's easy to throw up a page saying "Down for maintenance" for all the users, and then test a fully working one from the network behind the proxy.
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