You can convert a regular spell to a cure spell at the same LVL (if you're a good aligned cleric), but I don't think you can convert a cure spell to a cause wound spell... (well... certainly not with a good-aligned cleric). the single rule that I know that is conected to this is that if you cast healing spells on undead (with positive energy) it will hurt them, and vice versa.
I hope I helped...
