We did this just last week.
We'd originally made a PS for 2 at 6:15- and had a friend who was able to join us about three days before our PS. We called to change the party total, and had to slide the PS to 7:15 because they couldn't fit 3 into our original time.
We ended up keeping the original PS confirmation number, but they shifted it to a time when the 3 of us could get in.
I hope that this helps........