1. When u get the character switch ability u are on the balcony but still can not use it
   
2. Once u get the first vision (using cognition on the note in the kitchen table) u are inside the penthouse and idk why if u try to go to the balcony to make the switch outside the achievement does not pop up. I tried several times with this first vision (Cordelia and John) but it does not pop up.
   
3. So the problem I'm sure is the vision. You must have another vision to get it.

The vision that you can obtain in Cordelia's room (using cognition on the jewelry) also does not work bc Cordelia is sleeping.

When u are at the penthouse, go to the door at right to arrive at the office. There u will get another vision (use cognition on the envelope) to use with the switch ability. Get out of the vision and go to the balcony. Use the character switch there and the achievement will pop up.