I think there's also an unwritten rule that if you're swapping more than you're leaving on the robot it's not the same one so not allowed

It's obviously all down to the judges so if I changed 10kg of a 13kg machine I reckon they'd have issue - best thing to do would be to email whichever EO it concerns and ask!

As for the specific rules, it's just the heaviest setup that it's weighed with, as long as that's in weight you're fine.