That doesn't seem completely true. I could imagine default language syntax changing from one version to another, but still being modifiable via "from __future__

That doesn't seem completely true. I could imagine default language syntax changing from one version to another, but still being modifiable via "from __future__ import ..." or "from __past__ import ...", respectively. Unless the bytecodes of the VM are incompatible (which is possible), there isn't any reason py26 code can't talk to py30 code...