They changed most of the lore around the elves. I have ranted about it extensively lol. The only thing they really kept was that elves were born of Corellon's blood and the genderfluidy of the Seldarine.Okay I have to ask just how much of change did they make of the elves, dwarves and halflings?
Elves have always believed in reincarnation, but Arvandor was a reward. MToF however presents it as a temporary summer home, where elven spirits stay for a time, and then are reincarnated into elves because they are denied permeant residence in Arvandor due to what their ancestors (called the primal elves) did; some sided with Lolth, and as such, all are collectively punished. The pull to Arvandor and tie to Corellon remains strong, but reincarnation isn't a choice--it's forced on them.
There are other changes, too. I won't go in to all of them here--and don't get me started on the drow lol.