Okay...I thought this was just common sense.
SOMEHOW a couple people at the last locals I went to got the idea in their head that they are allowed to switch decks in the middle of a tournament. I kept trying to tell them that was incorrect and they want proof. I've spent about an hour looking but found nothing (of course, as I feel this shouldn't even need to be a question).
PLEASE someone give me some source that will settle this issue for good. I have never known any card game tournament to allow this, but they're claiming that if it doesn't say no in the rulebook then it's legit.
If somehow I'm wrong, I guess I'll just learn and adapt to that, but 100% sure I'm right.