I'm playing a guess game here, but try this:
In the carrier settings, turn this option to disable carrier:
This way, any order above the max weight set won't be able to use that carrier.
Then create a new carrier, set the weight rules starting from the max-weight allowed in the original carrier and set shipping cost to 0. Imagine they want to set the maximum paid shipping weight at 50Kg.
Let me know if it worked as intended.