XP

The change might seem small however it will have an everlasting impact. Soul Stones symbolized a fictional currency that could be earned by just being active. Thus there would be able to compare the amount that you are able to earn per day in comparison to how much it would cost to buy them.

After all your Soul Stones were converted into coins, you might have noticed that it's become much much harder to earn them. Now the only method left would be to invite new people.

This directly turned the currency from an earnable one into a premium one. This might have been a method that Webnovel decided to implement to make it easier to deduce how much revenue an author is actually bringing in.