Welcome felicitea!
No. Unfortunately the extensions are not "live". You'll have to create a whole new one, each time. However, there are probably some ways to smooth out your workflow. (There have been some requests for this kind of feature though, so you're not alone in wishing for it
)
The dialog offers a "live" preview, which you might find some way to use. You can't snap to the preview because it's not really a path (yet). But if you're in the middle of drawing the symbols in the diagram, and realize you need to change the grid, you can delete the old grid, and use the live preview to see how different configurations of the grid will look against the diagram. You'll need to set it up so that the grid preview is aligned exactly like the old grid.
Or you could probably come up with a way to use Layers. When you create a grid, put it on a layer by itself, when you reach a point where you need to try different configurations for the grid, hide that layer, and then create a new grid on a new layer.
Just a quick note about the extension dialogs. You might have already realized this. They can only remember the configuration of the last object they created. And you can't select an object drawn by an extension, and retrieve the configuration (settings). So if you have any that you might want to use again in the future, be sure to write down all values.
