Not true.
They have the authority to rewrite the rules up to the day before the convention.
This is the test to win and then they will move the goalposts.
Problem is, they risk losing the House and the Senate if they do it, and they risk losing the House and Senate totally two years later if they do it along with Governors, state legislature, judges, etc.
Can you site me the rule that says that and explains what it takes to do that?
They risk a revolt like they have never seen in their lifetime.