I've tried it and it dosen't work.

This is quite a mystery.

Or I must be doing stuff wrong. I've tried:
puting the compatility script above and below the blood mage, blood mage below and above the Blizz ABS
putting it in the blood mage script, in the middle and bottom
Am I still doing something wrong? (Gosh I have learn how to script to stop asking and looking like a total n00b)
Anyway I found a way to use a common event.
Grand Cross calls a common event that deals 50 or something damage to the guy who uses Grand Cross. No switch needed and you can use conditional branchs depending on the user's level, if he is level 50, it deals 95 damage. That's just my idea, though. I'm pretty sure you guys can do better.