I imagine the difficulty would be in ensuring that the train lines up exactly with the doors. In that cade, retrofitting would need to apply to the train and control system, not just the platform itself.
https://www.atlasobscura.com/places/14th-street-union-square...
Every station has lining boards that the conductor must check before opening the doors to ensure that the entire train is stopped along the platform:
So for a long time, drivers had to manually stop the train at alignment points. This did cause occasional delays and problems, but it wasn't unworkable. As a passenger you did sometimes notice that the train doors weren't aligned perfectly with the platform doors. Now days, of course, it stops perfectly every time.
They built a new line on second Ave with cavernous station mezzanines and couldn’t be bothered with doors there either. Track, signals, trains were all new.