A company I used to work for attempted to migrate once and nearly gave up, but then realised it was feasible (and not very expensive) if we did not insist on pr

A company I used to work for attempted to migrate once and nearly gave up, but then realised it was feasible (and not very expensive) if we did not insist on preserving the history. So the new revisions were kept in Mercurial and the old ones in CVS. A slight inconvenience for the first year, but after that nearly all of the interesting revisions were in the new system. As for the legacy systems, they already required us to maintain Windows NT4 boxes and a no-longer-supported C compiler, so the additional overhead of a second VCS was tiny by comparison.