I don't think anything important is going to happen

Really, if we assume that traders hadn't been withdrawing their money from these exchanges through fiat during this time (since otherwise your question wouldn't need an answer), and they all rush to withdraw their bitcoins once it's allowed, what will it change? They just move their coins from their exchange accounts to their personal wallets, and that's pretty much all there's to it. You seem to assume that they will try to sell the coins at other non-Chinese exchanges, but, first, why would they want that (if they didn't sell them at their local exchanges in the first place), and, second, how are they going to withdraw fiat from foreign exchanges they move their coins to?