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.
The new change to a free pa.s.s system also causes some novels to be way more desirable than others. A free pa.s.s can be used to unlock a single chapter. Thus a chapter that would cost 6 Spirt Stones is much less desirable than one that costs 12.
This change might have a major impact on how authors plan their chapter lengths in the near future. Longer chapters would result in either a slower release rate or too much filler.