[Tyrant]

I cast it with more gems than you used originally and thus overwrote it.This is always the way it works when the slots are full and you cast the same spell.

If the slots are full and you cast a different global it's not so clear. I think that the new one goes up against a random existing global and the higher gem count wins, but some people believe that which old global the new global opposes is not random. Some people also believe that the the usual 2d6oe opposed roll is used for global dispells and overwrites, but i'm nearly certain that that is not the case and the larger one wins automaticly.