I'm afraid I don't know any details, as I wasn't a playtester. The only info I have is one thread that Kevin Wilson replied to on BGG where he stated that the basic rule for how twilight worked (i.e, how the twilight marker moved) in an earlier version of the game was how it appears in my variant, and that it was removed because playtesters felt that it encouraged too much of a feeling of persecution.
You can even observe rules in the current rulebook that are obviously holdovers from the earlier system. I'd take an educated guess that the following rules were required as part of the old system, but with the new system are just part of the "rules bloat" that many people feel about this game:
* The "shift" bonus on the back of conspiracy tiles.
* Allowing characters to light or dark shift as part of fights.
* Allowing cards to be discarded to pay twilight costs.
* Twilight discounts for matching plots.
* Any game text that allows you to increase or decrease the cost of a card.
With the current rule system, all of these rules are unneccessary, as having your twilight marker at any one point on the twilight track is equally beneficial as having it at any other point (the only differentiating factor is the number and type of cards you have in your hand). With the old/variant system, these rules would become much more important, as they are in place to prevent any player from "locking in" as completely light-shifted in order to prevent any dark cards being played on them.
(There are also a couple of Raymond's cards that highlight the old system once being in place - they allow you to manipulate your own/your opponent's twilight track, which with the old system is a big bonus but in the current system is next to useless).
The variant I came up with changes the rules back to something close to what I imagine they were originally like. The only two major changes I've made are to have detectives have their own dark cards drawn when entering seedy locations, and to have detectives light/dark shift when entering seedy/ritzy locations. This is to further prevent the "locking in" that I mentioned earlier.
The variant can be used completely independantly of the others. Enjoy!