Not true. They changed the rules in August. I’m pretty sure all the candidates had declared by then.
You are correct. I was wrong.
It's even possible that the rules were changed to stop Trump, though it's unlikely they intended to help Cruz.
That still doesn't negate the fact that the rules were set LONG before the event happened, and in plenty of time for Trump to take advantage of them better than Cruz did.
If he becomes President, Trump will have bigger, smarter, tougher enemies than the leaders of the Colorado GOP. Putin comes to mind. Trump needs to prove he is smart and can build an effective organization of good people around him, that can accomplish something in a field other than real estate. So far, his campaign seems to indicate the opposite.