
Trivia: In the evidence depository in the beginning you can see Jason Voorhees' hockey mask (Friday the 13th), Leatherface's chainsaw (Texas Chainsaw Massacre), Michael Myers' mask (Halloween), and Freddy Krueger's glove (Nightmare on Elm Street). (00:00:45)
Answer: There's two possible explanations. First, and the most likely, is that this is simply what's referred to as a "retcon." (Short for "Retroactive Continuity") A term to describe new information/rules/backstory/etc. that are introduced in order to alter the path of a story, or impose new ideas into a narrative. In this case, the series creator and writer Don Mancini needed a reason for the characters to go on a road-trip, hence he created this new idea for an amulet that Chucky need to obtain. And basically, Mancini himself has admitted that he will often change the rules for the series as needed from film to film based on the story he wants to tell, which makes this the more plausible explanation. The second possible explanation is that you could argue that the amulet will allow Chucky to transfer his soul regardless of how long he's been in the doll's body, surpassing the time limit imposed in previous films. But in all seriousness, the former is the more likely explanation. They just needed a new story-element to justify the road-trip aspect of the story.
TedStixon
Both answers work for me. Thank you.