My understanding is that for the first season of building at least, the existing Main Stand is unaffected, as building works will only take place BEHIND the Main Stand for the first year.
There might be different entrances though, and some of the 'lounges' that extend to the rear of the stand might be demolished during that first season, meaning it's a less swanky experience
I am curious at what point the club 'chops off' the front 6 rows of the Whitehorse Lane Stand - there will be hundreds of season ticket holders there who will need relocating