I work with computers, and with databases, since decades.
And when I read something like above, then I have my doubts.
How big can a database for a forum be? Not huge!
If it is so big, can't all old data, i.e. everything until last month or last year, be transferred first, and only a little new data later offline?
If you do it right it isn't that complicated and it isn't that time consuming.