> How did OAI handle the 1.0, 1.1, 2.0 transitions?

From the OAI FAQ - they didn't:

"Is version 2.0 of OAI-PMH compatible with earlier versions?

OAI-PMH version 2.0 is not backward compatible.  Results from the
version 1.x experimentation period indicated that a number of
fundamental changes were necessary, and maintaining backward
compatibility would have resulted in a compromised production release.
We are committed to strongly consider backward compatibility issues in
future releases."