I painted some frames and the brush was too thick. How do I change all the brush stroke widths in a single frame?
The only way to do that with a Brush is to resize the actual art to smaller, and the brush line will thin out. You can do that by selecting this:
And then dragging that grey ribbon to the last frame and then click drag over all the frames, Then resize.
Then you can increase the size of the screen dimensions which will essentially do what you're asking although granted it's a workaround at best. That's unfortunately pretty much it as only pencil lines can be changed after the fact.