What changes?
I've read about a bunch of little changes, but nothing absurd.
okay so i found a site that documents stuff:
https://www.theonering.com/i can say just in general,.. because of the thickness of the books a lot of content had to be cut. BUT,... there is a crud load of content that was created out of thin air. some of it feels possible/probable and just not discussed in the books but some of it doesn't seem to fit at all. generally speaking the movie made up a lot of scenes or changed details about scenes to make the movie more action oriented and dark. i think this especially hurt the "rise" of the movies,.. encounters went from mild to intense as the story progressed. a lot of make up crud is in the first movie where they wanted a more exciting movie than the story that was written: orc pod, lurtz, various black rider encounters, the saruman and gandalf wizards dual.
..but a few key details that really ruffled my feathers.
Faramir was depicted as cruel and greedy by capturing frodo and (the part i was really mad about) taking frodo back to minas tirith. none of this happens in the book. faramir (the brother to boromir) is very gentle natured in the books and assists frodo without the need for explanation on his quest. in the books, his character was made to be a counter-point to boromir whom betrays frodo in the first book. that whole scence where frodo confronts the ringwraith really pissed me off. all this fabricated content end in a completely unsubstantiated 180 in faramir sending frodo back on his quest. it's like 30 minutes of completely fabricated BS that is completely counter to the themes established in the book.
in the first book they cut out tom bombadil. it was like half of frodos journey from the first book and pretty cool. why cut what's awesome to put in crud?!?
elves (other than legolas) at the battle of helms deep. way to completely fudge up the entire back story of the elves bumb... really there was a lot of changes to the elves to make them feel more important then they were. the elves were extremely important in the hobbit but the theme in TLoR was fading glory.
one change i do approve of was removing the scouring of the shire. the final book "ended" and then kept going for like another pointless 100 pages on pointless crud. it really put a sour note at the end of some excellent books.