Make major modifcation/s to online forum without major interruption?

[akm]

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Part of basis for question is at...
http://community.mybb.com/thread-115934.html
...how to add a /forum2 on the same server with another /fourm1.

That question/answer was prompted by a question (in the grey cells only) of how to replace an online forum with a new significantly modified version of the same forum without losing any of the significant data (like user registrations, threads etc)?

Is it to create a duplicate forum, make major modifications to it, then 'replace' the old forum with the new modified forum, and how to do that without losing any of the significant old forum data...  ?

Where to find thoughts/opinions/definitives ?