My team is moving from Visual SourceSafe to Subversion soon, while developing/supporting a legacy project in Visual Basic 6.0, so I have a couple of questions:
Depending how much you're planning to do on these legacy projects I would consider not switching.
I would really advise you to switch to SVN. I know of a few projects that lost source code because the VSS database became corrupted.
I think there are tools that perform the migration from SourceSafe to SVN. (Yes-- a quick Google search confirmed it.) That way you wouldn't be losing the revision history.