Raquel, there is a reason for using the matte finish that you may not have considered. It shows small marks and defects less. A wide, shiny expanse will show up every tennis shoe abrasion and high heel mark. These will still be there with a more matte shine, but they will be less noticeable.
By the way, polyurethane does not stick to itself well, like some other finishes do. You will want to lightly sand the floor and clean up all the dirt, dust and sanding dust before you put down the new layer.