Yeah, you don't need to repack them at all and can just reset your content files to restore them to their vanilla state, then leave your whole Content folder alone. The
list I linked to check if your XNB mod already has a CP version also has some info about why, and more links for more info.
If you are creating the mod yourself from scratch, there are many sprite mods already available that you can use as examples. The linked alternatives page has several older conversions, but you can also just find them browsing mod sites, like
this one that replaces Abigail's portraits and sprites with different ones.