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Ah... thanks for that clarification. So even if a supermajority of top witnesses made this change, if Stinc isn't on board and doesn't update api.steemit.com for instance it wouldn't work?

Correct. Transactions go through the front end nodes first. For steemit.com users (and indeed many other app users) that means nodes run by steemit.com. If those nodes don't pass through the transactions, witnesses never receive them and they don't go the blockchain.