[thejeff]

No. Casting a different global while the slots are full will try to overwrite one of the existing ones.

Dispel allows you to pick which one you want to kill.

There are a number of reasons you may want to Dispel instead of replace. As you say, you may not want the effect up at all.
Also Dispel is fairly cheap and low path requirement, so given the gems/caster you have available you may be more likely to succeed with it.
It may also be easier to get donations from other players to Dispel a problem global than to transfer it to yourself, unless you have a very strong alliance.