It's not a problem. I had the same worry as I had renewed my passport in my married name, but not got around to changing my SS number. I was able to get little one's stuff with no worries. I don't know why they say that. It must mean something different than it seems to.
By the way, you can do it while your there for your child. I just had to go to a different window and show them my marriage certificate or something like that. I didn't have an appointment, I just asked someone while I was there and they gave me instructions, so now all my info is up to date.