What determines whose Global spell sticks?

[SurvivalistMerc]

You guys are right in that casting a global spell can take out another global spell. That's in the rules.

But doesn't that happen only when all global spell slots are occupied?

My assumption...and perhaps I should not have made it...was that there were only one or two other global spells operating at the time.

Does anyone know whether you can "dispel" a prior global spell if there are additional slots left? I thought that you couldn't.