The phasing option applies to the whole curtain wall as 1 entity true. You will need to nest the exisiting openings and panels which you want to demolish...ie:
- Select element/part
- Unpin
- Use a blank panel instead
- Place a new Curtain Wall in the resulting opening and use either window/door or panel identical to the existing
Now you can demolish phase those nested one and construct new to replace them...That has been the case for renovation projects where you need to partially phase an existing curtain wall
See following screencast by Paul Aubin...it is old but rules are still applicable