In a June '18 update on indiegogo, I read this,
"In other news, we'll be working on a second sprite set that matches the game's CG style. Players who prefer the original set will still be able to use it, however. "
https://www.indiegogo.com/projects/signed-and-sealed-with-a-kiss-visual-novel#/u...
If this is still a thing, I'm assuming the new sprite set is ust to keep continuity between the sprites in-game and the CGs :)