Was able to convert my Xbox One world, 285mb, 2 times but half my town was replaced my new land. After this I haven't been able to convert my world, it reached 80 to 90 % and then the game crashes making me have to hard reset my console. Spent over 500hours in this world, no way I'm starting over, please fix this.
So I finally caved and decided to remove all my chest, around 300 of them, except one double chest where I stored my most valuable things and it did convert. It is not perfect but I can replace all of the stuff I lost it will just take time. My world was created on Xbox One and the only things I removed where chest, still had a bunch of farm mobs, picture frames, armor stands, redstone things and so on.
Just a warning, you don't seem to be able to reset the End or the Nether so make sure to do that in your OG save before you move it over.