You would not have to reset the config, and doing it remotely also should not be a problem(notice the
should).
There is always a chance of something going haywire during an upgrade so whenever I plan a remote f/w upgrade I always make sure I have an opening in my schedule first thing the next morning...lol. I've done 50+ remote upgrades to various zywalls and the only issue I can remember was self-inflicted because I failed to thoroughly read the release notes before upgrading a ZW110 from 3.1 to 4.0...
JP gave some good advice in a recent thread which applies to you as well:
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Can you skip a firmware version on a GS1900-16 switch