Basically a sequel introduces a rule or lore that is seen as incredibly important for the entire franchise, except this rule or lore was never established in the original work and in fact may even contradict it.
Grease 2’s entire plot revolves around the idea “Only T-Birds Can Date Pink Ladies and vice versa” as the main character a non-T-Bird wants to date a Pink Lady and gets in massive trouble for it. In fact he’s told not even to look at a Pink Lady, it’s just that dangerous for him.
But then this rule is literally NOWHERE in Grease 1. In fact the Pink Ladies and T-Birds do seem to date whoever they want over the course of the movie and seem largely independent of one another.
The well-regarded sequel to Highlander clarifies the origin of the immortals, and was immediately recognized as an important and faithful contribution to the lore.
I’m not sure whether this is the kind of thing the OP is looking for or not. Those are reveals or plot points appearing in sequels that shed new light on, or change our understanding of, the original. But they’re not incompatible with it; they don’t change the way the world works. George Lucas may or may not have intended them from the beginning, but there’s no reason he couldn’t have.
If it does count, then so does that old magic ring that Bilbo Baggins found in The Hobbit. In the sequel, we learn some new and highly significant things about what it is and how it works. But I don’t really think it does count: it (and various other bits of Middle Earth lore) is something we didn’t know about in The Hobbit, but it doesn’t contradict or detract from the Middle Earth of The Hobbit.
This is a decent “well, kind of” example, I think. Especially if you only read the first edition, where Gollum loses the riddle game then shows Bilbo the way out, fair and square, no big deal. In that case, you well might read the LotR and react, “Wait, what? That ring is the cause of all this ruckus?”
Only after it was published did Tolkien ponder what such a ring would imply in terms of power and backstory, and he retconned the tale so that the first edition had reflected a lie on Bilbo’s part—if only we had known the truth before it went to print!—concocted by our beloved hobbit to provide cover for an evil ring already exerting its sinister power. Later versions reflected the “true” story, with Sméagol’s treachery and his horrible attachment to the ring depicted.
So, I’ve check with the judges, and they’ve ruled that this is a fair example. 25 points to @Thudlow_Boink!
In Star Wars, Obi-Wan teaches teen Luke how to use the Force - and offers to teach Han (suggesting that anyone can learn it). In the sequels/prequels, ability to use the Force is rare, and can not be taught to someone old enough to be “too attached” to the world. You’d think a smuggler with close friends would be more attached to the world than 6-year-old
According to Memory Alpha, In Star Trek: TOS, exceeding warp 10 was considered generally unsafe, but not physically impossible. That page lists several examples of non-Federation ships said to be capable of speeds in excess of warp 10.
In TNG, warp 10 is now a physical limit, impossible to exceed. You can get to warp 9.99, but never exceed warp 10.
There was a Warp number recalibration between TOS and Voyager - for some reason they are just a scale where Warp 10 = infinite speed in the Voyager era, when in the TOS era, Warp 10, 14, etc, were just really fast speeds.
