Other mistake: When Tiff picks up Chucky to throw him into the crib, you can tell that the Chucky doll is inactive and not on or alive. (00:21:20)
Other mistake: In the beginning, when Tiffany has just had her soul put into the bride doll, she is reading the "Voodoo for Dummies" book and Chucky tells her that the page she is looking for is in Chapter Six, page 217. When she flips to it, the camera shows the top of the page before panning downwards. On the top of the page it very clearly says "Chapter Eleven". (00:31:45)
Other mistake: At the end, when Jade opens Chucky's coffin, there are several large rats inside, despite being sealed and buried under 6 feet of dirt for over ten years.
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.