You could go to the cruise meets board for your scheduled cruise date and find a fellow passenger on that board who has 1st dining and will link their dining with yours. I got dragged into 1st dining that way when I needed to change from late and there were no openings....
or wait 'til you board and ask guest services the best way to go about it...or, eat at Topsiders(?) the first night and grab an opening as things shuffle...They always do....Many options, room service, etc.....don't fret....