Changing doctrines is not a reflection of New Light ™ being shown along a path. Changing doctrines is a reflection of changing paths completely in the hopes the light is better on the new one.
Think for a second how an honest person would deal with a situation where they were literally looking for something and the light was literally getting brighter.
An honest person would say:
(Little bit of light:) "Hey, I see something. I don't know what it is or how far away it is. I can't tell if it's safe or dangerous, but there's something there."
(Light a little brighter:) "OK, whatever it is is so tall by so wide. Still can't see it well, but it looks to be this many feet away."
(Light all the way on:) "Hey, I can see exactly what it is now and can give you all the details."
Someone once said a watchman on a tower wouldn't be very good if he wasn't extra cautious by sounding a false alarm when he wasn't sure what was coming, in the interests of protecting a town. But that isn't how a watchman (or, at least, a good one) does his job. (And how many false alarms is he supposed to sound before he gets fired?)
Watchman: "Hey, there's something out there. I can't tell what it is, but I can see something is going on."
A little while later: "OK, I still can't tell what it is, but it's headed this way and it's big."
Even later: "It's close enough to see it now. Call the army, it's a band of invaders!" or "Don't worry, it's just a stampede of elephants."
But that's not how it works with "new light" in the Org because the one on the path is neither "discreet" nor honest and humble. Additional knowledge doesn't contradict older knowledge unless what's passed off as "older knowledge" is really supposition, conjecture, or just plain fiction. If the path of the righteous gets lighter and lighter, there is only progressive and more detailed knowledge about what has been taught previously. Completely changing the teaching means you decided you were on the wrong path and didn't like what you saw.