I recently did this exact thing, I originally booked as a solo, no solo discount. When I added a person, the entire cruise was repriced (it increased the price by a substantial sum) and I actually lost some of my original onboard credit. Not a fan of this, seems very unfair to a person who books far in advance to take advantage of pricing. Next time, I will not book as solo, I will go ahead and book a 2nd person to be named later.