As the days, weeks and months have progressed, we have been given more and more information based on the Village and Pillage update in betas, snapshots, and information by developers. For existing survival worlds, you might be worried if you cannot prepare for the update as you already have a world. But that is not the case. All existing worlds can receive the new features to some degree and it will come at a cost.
Changes to existing chunks
For existing villages, you will not get the new village look as the village is preloaded and there is no way to revert it. That being said, the villagers will not change either for the same reason as the villages which can have its positives and negatives. If you have a breeding system, it will break after this update as the way villagers now breed is with beds so there go those MANY hours of collecting wood for doors.

Changes for new villages:
The new design for villages will come in the form of a design overhaul. Each and every building now has a new design and layout for it. The block palette has remained the same with a few new ones being introduced.

Changes for new villagers:
The villagers will have major changes in the village and pillage update. Firstly, they are getting a new design overhaul so each and every village has its own unique piece of clothing. Secondly, their UI has been given a HUGE revamping making it easier to trade with them. You can see trades up to the next level and multiple trades at once! There is also a progress bar to help you track when a villager can upgrade to a new level.

The Village and Pillage update is going to change how we look at villages forever. It will both benefit us in some ways and have us at a disadvantage in other ways. It will hopefully revive the way we use villages and villagers in our worlds and allow us to protect them more.