I have worked with many source code management systems and experienced many transitions from one type of system to another.
Sometimes, much time is spent on keeping all the history intact while performing a migration. For some systems, this effort is well spent, such as for venerable free or open source projects. Emacs, for example, has made several such transitions over the years.
For many organizations, keeping the history is not worth the significant expenditure in time and effort. If an older version is needed at some point, the old source code management system can be kept online and referenced. This includes migrations from Microsoft Visual SourceSafe (VSS) and ClearCase.
Some migrations are trivial though, such as moving from Subversion to Git. In these cases, historic accuracy need not be sacrificed.