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.
When digging through legacy code it really helps to have all the history and blame. SVN is miles better than VSS, but you'll be losing the history when you switch.
If you're going to be a lot of ongoing development in VB6 then it may well be worth switching to SVN, but if you're going to be doing that much going forward is it also worth reviewing the project?
I have a similar problem, only the legacy projects are in Delphi. Were they in VB6 I think I would consider 'upgrading' them to VB.Net, just for maintainability.