Well if the module you bought is d20, its pretty simple. Most of the monsters stay as is except for their initiative modifier which now equals their base Reflex saving throw plus their Dex modifier.
If the creature has a natural armour bonus, that becomes its damage reduction or "armour".
Its Dodge defence value is equal to its Dexterity modifier plus either the dodge bonus of the barbarian class (for a good reflex save) or the dodge bonus for the scholar class (for a bad reflex save).
Also, if the creatures has a bunch of zappy magic abilitites, I'd remove most of them and have them keep 2-3 abilities that match the flavour of the creature best.
Hope that helps.