r.e. moving from SVN to HG - I've had success with the various tools [1] in converting entire histories to mercurial [2] (it was about 400 change sets) and also have been maintaining an SVN/HG dual repository for a wordpress plugin.
1. http://mercurial.selenic.com/wiki/WorkingWithSubversion 2. http://hg.errant.me.uk/eventscripts-xa/changesets