My take on this debate, since it seems that we all get to chime in
Casting a new global and overwriting one when there are 5 globals up shouldn't constitute a breach of a NAP. Casting a global that is already up, which will always overwrite a single individual's global, would be. Why?
Well, dispel seems to be pretty uniformly agreed to be a breach of a NAP. It targets an individual to their detriment. Overcasting a global (in this case, Samhaim's Mother Oak replacing Finalgensis') results in the same thing, but with added upside for the overcaster. It still targets one individual with whom you've presumably reached an agreement and can substantially hurt them, only this time you profit more directly from it. If someone that I have a NAP with overwrites my Well of Misery, for instance, I'll definitely consider it a breach and declare war. (Just as a heads up, everyone!
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That said, considering that Samhaim says he didn't consider it a breach, it brings to mind something I read when I was first researching diplomacy for MP Dominions: if something isn't clear, spell it out when you agree on the NAP in the first place. Misunderstandings can happen, after all, and it's better to get all that out of the way at first. In any case, that's my piece. Considering this is a no-rules game newbie game, we should all take it as a 'live and learn' experience.