Which is utterly trivial (I've done it, seriously, it's not the big deal you seem to think it is, aside from the obvious difficulty of particularly large repos), and is not conceptually different from what's necessary for editing git's history, except that nobody can tell you've done it without comparing the "new" repo to the old one -- and under svn's internal model, no one but the server will normally have a complete history.

With git's model, not only does everybody have the history, but the commit ID themselves are your insurance against tampering. You effectively validate that history every time you sync with another git repo.