Perhaps I'm not doing this right, but I believe the current method of redefining a dimension type requires you to save a custom setting using the exact same name of the existing type you want to modify. Only then does AC think you might want to tweak an existing type. There is not an option to reuse an existing name. If you enter the name of the dimension type incorrectly, then you get two types with similar but different names. And it's been this way for decades!!
I recollect that redefining an existing pen set worked this same way until a few years ago, and even now its a bit clunky.
If I'm wrong about this, and there's a much easier/smarter way to redefine a dimension type, I will be happy to correct my understanding.
Mac Studio, 32gb ram, ArchiCAD v26 (Apple Silicon) MacOS 13