Hi,

I'm wondering about the differences between the aircraft shell kits  (upper & lower) of the MG-1S, MG-1SA and MG-1P, can they be used  interchangeably?

Posted in the products section as per suggestion from another user.

There are no of differences between them. The major one is obviously name. DJI have a well detailed post about them, here you can find the exact difference.
https://www.dji.com/products/compare-agriculture
You can also check this https://talosdrones.com/product/dji-agras-mg-1p-sprayer-drone-bundle/ To buy the product. For any queries you can ask the respective authorities.