Definitely that is the way. By "duplicating" I meant exactly this: to make a completely independent, not publicly visible copy of the existing web site, where we can play around until it's perfect.
Attention: we'll have to take into account, however, that the existing web site may evolve meanwhile we're playing around with the copy, so before finally swapping in the new one for the old one, we'll have to make sure that any content that was added to the new one is reflected on the new one as well. We'll how see how exactly this is best achieved.

About the first steps: according to [1] it's not really a copy of the existing site that we'll need but an EMPTY Plone 3.3.x install. When IOhannes has the time to set it up, we're ready to go.

[1] http://plone.org/documentation/manual/upgrade-guide/version/upgrade-pre-2.5-releases-to-the-latest-release


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