1. Don't publish under the ammended GSL that says you can't use any other license.
This is prediction, and assumption, and should probably factor into a business's decision, but has nothing to do with the wording of the license.
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Can they?
You've accepted the terms of the license, one of those terms being that if they change the license, use of the changed license implies you accept the change.
If they remove that part, then there is nothing implying you accept the new license, and it would stand to reason that they would have to have you re-issue the acceptance letter.