@BenWeld welcome to the forum
It’s not easy to unbake the cake. Using a program like g code ripper you can create a dxf file from a NC file. There is a catch, dxf created by g code Ripper will be almost the same as the original minus the kerf width.
If it’s just the cut height you need to modify you can manually change the g code. There are different G-Code viewers some of which can find and replace certain instances in the code. ( Like the cut height throughout the document)
There is a link to g code Ripper it this topic.