How to cut this headboard down from King to queen?

a king bed is just 8" wider than a queen. Rather than cutting it down could you purchase the mattress extenders and use them in the reverse?

The hard part is going to be to get the cross bits to fit the posts without having to fill the old holes and redrill new ones. Ideally, you would cut it at a level that the new bit would fit into the same hole by using a string level to line up the existing holes with the exact place to cut. Eyeballing it, I'm afaid you would wind up with a single, not a queen. A couple of things to try: (A) You could in fact cut a few inches off each side, then fill the old holes, and redrill new holes. The whole thing will need to be painted. (B) You could live with the mis-matched headboard and matress size. Set the new bed-stead up, and either (1) attach a wooden extention so that the over-sized headboard will fit or (2)attach the headboard directly to the wall with clips or straps, and center the bed on it. I would likely go with (B), (2) with clips or straps.