The Biggest Change The Rings of Power Made to LoTR Canon
The Lord of the Rings: The Rings of Power is not known for its care toward The Lord of the Rings book canon.
In fact, you can probably hold a competition to determine the biggest deviation from canon in The Rings of Power, with quite a number of contenders. In fact, some of the fans just compiled a fairly long list of such deviations.
The smaller ones including Gil-galad having enormous authority – both worldly, and, more importantly, spiritual – he did not possess in the books; Palantiri gaining the ability to peer into the future they did not have in the books; and Elendil's status in Numenor getting much reduced from the highest nobility to which he was supposed to belong. These can be easily missed by people who have read The Lord of the Rings, the Appendices to it, and The Silmarillion long ago, or skimmed through the latter two.
The bigger ones include Finrod dying in Sauron's dungeons under quite different circumstances which had nothing to do with hunting Sauron after Morgoth's fall and everything to do with Sauron being Morgoth's chief lieutenant at the time; Galadriel's husband Celeborn going missing and presumed to be dead; elves having legends about ancient times like men do (instead they have eyewitness accounts – in the books Galadriel could just ask her granddad what the beginning of elven history was like and Elrond had personally witnessed what actually happened to Silmarils).
Then there's also the whole mythril snarl, which not only ties together the two things which had nothing to do with each other originally, but heavily redefines Middle-Earth's metaphysics and elven nature; and the tweaks to the story of making the titular Rings of Power, which create a bunch of plotholes, starting with Sauron already getting exposed and chased out of Eregion while there are still sixteen rings – and specifically those in crafting of which he was supposed to participate – to make.
But which one gets the prize for being the biggest change?
Well, that depends somewhat on personal opinion, but I'd nominate all the things which showrunners did to the timeline. And what they did? Long story short, they have compressed events of over a millennium and a half – that's roughly the equivalent of time from the fall of the Western Roman Empire to the present day – into a few years.
Yeah, yeah, they needed a way to keep mortal and immortal characters on the same wavelength – how about not trying to stick both the forging of the Rings and the Fall of Numenor into the same series, or employing a timeskip, then? And given that we've seen the Balrog awakening, the series might attempt to include even events of the Third Age, again many hundreds of years distant.
At this point The Lord of the Rings: The Rings of Power is a completely separate continuity from The Lord of the Rings, and we can only hope that everyone remembers this, before engaging in Internet discussions.