One thing to note about older editions (pre 3e) is that they are very downward compatible. You can pick up a Basic module, like the classic Keep on the Borderlands, and use it for a AD&D 2e Players Option group with very little need of conversion.
...to an extent. Between 1e and older version, this was fairly true. There's a slight increase in power with 2e... that ramps immensely when you add PO.