SVN is the centralized repository system that many developers should have been familiar with. Mercurial (Hg) is one of the latest distributed repository system that is gaining popularity, besides Bazaar and Git. TortoiseSVN and TortoiseHg are GUI tools to manage SVN and Hg respectively in windows. That said, this note would apply mostly to windows users.
I would demonstrate two options to the migration but the outcomes might not be the same and I would detail the difference of each.