You can change the design on the interior walls of already existing apartments. So the outer walls stay the same, the interior you can customize to liking.
And the penthouses you can create houses on, like you would with normal homes, but over there it's on top of another building.
If you mean, can you build apartment buildings on residential lots in other cities, the answer is no. As per above response, you can remodel most apartments and the penthouses are treated as a residential lot so you can build anything on top of a building but you can't build an apartment building in any other city and have it be recognized as apartments. To my knowledge, only the pre-built buildings in the city that came with the city expansion pack can be recognized as apartment buildings.
I was so looking forward to building apartment buildings. Just a little disappointed, that's all. Just voicing my opinion.
