The manual says "The nice thing about SOI is that it can be traded, reduced, or modified by events and diplomacy (normal diplomacy or as part of a Crisis resolution), so it’s never set in stone, as it takes into account the game’s dynamics."
One specific is that SOI is not changed but PP received are halved if not a Protectorate or better.
The manual does say strong economic activity may change SOIs, which suggests that even negative SOI areas that are populous and resource rich could with effort develop and improve to positive SOI.
Any input on seeing (or not seeing) SOI level changes for various reasons?
Thanks!