Depends on how the art is made, some are "modular" meaning that multiple pieces come together to form a scene and when the art is made like that you can easily do any changes you like and in fact, a lot of games in this forum use this method to ease the development process.
So nope, not everything has to be redone.
You're right, not everything has to be redone. It would have to be done in the first place, though. Like
j4yj4m said, the assets don't draw themselves, neither does programming and posing do.
You might notice that it's not only belly and clothing. Also arm poses would need to change for instance. Not to mention dialogs. It would not happen due to how the story is done (i.e. MC not being able to boink them this early), but imagine a pregnant Roxxy winning the bikini contest (would be hot, though), or a pregnant Crystal doing the catfight in the prison cell. Those events would need to be re-written.