By the time you reach level 7, you have 8 different single beds, and can mod any of them. You just can't use the higher level beds until you get to the higher level.
Initially I modded both pink and blue single beds to beige bedclothes and a bookcase headboard and stuffed 6 of them into the green room, because I had a big family and they were exhausted all the time. That's when I discovered that the beds are pink and blue, not girls' and boys'. The kids don't care what color the bed is. So I can mod one of them to something else. I am just stuck with two choices until I have a few more generations under my belt.