It's possible, but it's going to be a huge project. The sanding will take a long time, and you may still have uneven coloring afterwards.

I stained a light oak floor a dark walnut and hate it!!! You can see where the stain didn't take on the outer edges where we didn't sant enough. Just want to rent a sander again and remove the stain and go lighter. Is this possible now that I stained it dark?