[Alpine Joe]

I'm not entirely clear on how overcasting global enchantments work when you are casting another enchantment. Say I am in a situation with all the global slots filled and I try to cast earth blood deep well. Will my casting replace one of the existing enchantments, even if none of them is Deep Well? Does it compare the raw number of gems used to cast the two? Do I have to cast dispel first on one of them?

I checked the FAQ and manual and so far I can't locate the answer.