Unfortunately, they're right - if the plot is down, and the game saves, you can't change who moves in. If you think you have an inkling that someone might be arriving, what you can do is start a new character file. Just name yourself, go through Rover's intro, arrive at the station, and go wandering around looking for the plot. If it's who you want, great, if not, hard reset (home screen, close software, reopen). The game doesn't save new characters until you get your TPC - so make sure you do that once you get your desired move-in. It's a complicated and sometimes frustrating process, you can decide if it's worth the effort. ^^