I have realized that a long time ago, but I'm not sure how correct my hypothesis is.
However, I've come to the following conclusion:
Ever since the star rating rebalance, every map that had a leaderboard got adjusted. However, due to the sheer amount of Graveyarded and Pending/WIP maps, they did NOT get properly adjusted (Most maps show a difference of ~0.01 - 0.05* anyways). The only way to adjust them is to update them, by which they'd get recalculated and adjusted to the new difficulty values. This is why updating some very old Graveyarded maps might heavily increase or decrease their star rating. I guess these maps are slowly but surely getting updated as well now.
(As I said, just a hypothesis, but I think that might be correct. I've realized that back when I played a supposedly 5.5* Graveyard map, only for Bathbot to tell me that the map was actually supposed to be 6.3* after the rebalance.)
TL;DR
If you didn't update your map since the rebalance, it would've stayed around the same star rating as it was pre-rebalance, but it seems like these maps are being adjusted now as well.
If you updated an old map after the rebalance, it would automatically be calculated with the post-rebalance star rating and adjusted to that value.
If you submitted a map after the rebalance, it will change nothing, since it's already at the post-rebalance star rating.