Can you put a matte polyurethane over a semi-gloss polyurethane to make it a matte finish?
Yes (with some prep), however, you can try lightly sanding with 600 grit wet/dry sandpaper.  Use it lightly wetted. Buff dry with a soft cloth.  You may find this is matte enough.. DONE!

If its too matte (flat) you are now prepped and ready to recoat with satin or matte.