New tree or just a branch when changing languages?

Assume a project was written in .NET 1.0 and you want to make a new
version in .NET 3.5, but you still need to maintain the version
written in .NET 1.0. Is that even possible with VSS (or other tools,
for that matter)? Am I right in thinking that when you're changing
the language (which you're essentially doing going from 1.0 to 3.5)
that you really have two wholly separate codebases at that point?