As far as trailers go, put the "old film" filter back on top of the gameplay demos. Problem solved. Maybe remove the pure white borders, or at least decrease the thickness and make the it more of an eggshell white.
The in-game creepiness has decreased massively IMO, but not because of the art style change. The main issues are the players and the skin system.
When any player first started, Don't Starve seemed a lot more creepy because nearly everything was completely unknown. Everything was scary because it could seemingly kill us at any moment, and you had to interact with these potentially dangerous systems. Forests were scary if you didn't know how treeguards spawned or how to deal with them. Pigs were scary because once in a while they'd transform and try to murder you, especially with Base DS's unpredictable moon cycle. Tall birds were weird and scary and chased you forever. Beefalo could suddenly turn hostile. There was an enemy in the swamp that could pop out from beneath the ground. There were robots that attacked you on sight. And finally there was a terrifying giant that hunted you down in the middle of winter.
Then, we got better. We learned how to kill everything with minimal losses to ourselves. We learned where to get food. We learned why some creatures would or wouldn't attack. We learned to spot tentacles. We learned how to not have deerclops destroy our base. The game became less scary when we knew what to do and could approach the world knowing what it had in store. A new, solo player would probably find DST today just as spooky as you did when you first picked up DS.
However, there is one new addition in DST that I believe completely destroys immersion: The Skin Drops. The skin drops completely break the "feel" of the game every hour by asking you to "click here for your prize". It is a reminder that you are not a pyromaniac orphan, bereaved child, over-the-top method actor, or cursed lumberjack. It reminds you that you are playing a game in the 21st century. You might have been experiencing a death defying adventure or an uncompromising game of wilderness survival, but now it's time to play dress up. So drop what you're doing and get to a science machine because you don't want to waste any potential skin drops do you?
I understand completely that skins are necessary to the longevity of the game and keeping the servers running. But it still is, in my opinion, the greatest problem hurting the game's feel.