Is it possible to change the brush type of an already drawn grease pencil object? For example, I drew a line with the airbrush and I'm happy with the shape of the line but would like to have it drawn with the ink pen instead. Is this possible?
It's possible but you don't have much control. Here is how you can do it.
First Select your grease pencil object and go into draw mode and select the brush type you want it to be redrawn with. Then go into object mode and select your grease pencil object and right click and select convert to bezier curve from the context menu. Select the generated bezier curve usually named Lines. then right click and select convert to grease pencil object.