Some weeks ago people noticed that the sets that use the new worm gear started shipping with the old one, and their online instructions were changed to match. These are the affected sets:
• 42023 Construction Crew
• 42026 Black Champion Racer
• 42027 Desert Racer
You can see below an example of the change in the online instructions, for the 42023. On the left is a version not available any longer from the LEGO site, and on the right is the current version.
Some days ago an explanation came from the CEE Team:
The element has been pulled back since it didn't fully live up to the expectations.
Durability is not as the LEGO Technic team would like it to be, it can get overloaded/overworked too easily for their liking
This could explain why the 42028 Bulldozer uses an old worm where a new one would be expected: it probably was a change late in the production process, but early enough to occur before the set entered the market. I can see the reason for the pull back, as the worm’s driving axle can’t fully go through it, there’s the possibility of torsion forces breaking the new element apart in case of a stalled gear.
Still, the new worm has at least one big advantage over the old one: as it measures exactly 3 modules long (instead of the old one’s almost 2 modules), it produces a lot less backlash. Therefore I hope we will still get a renovated version of this new worm (like we did with the large linear actuators), and that this isn’t the last we see of it... but for the moment this is still known only in Billund.