The two big semantic changes with the long grass in Oort development is the addition of varying height long grass (not just a visual thing), and that the long grass is now driven by the grass blocks themselves, rather than long-grass blocks that are put on-top of the grass like flowers.
This means changes to the world builder and generation process!
These changes boil down to there no longer being a LONG_GRASS block at all, and instead of adding a smooth-decoration in the world builder that produces patches of long grass blocks, we instead use a GRASS substrate, which allows defining extra parameters and noise functions that control how tall the grass is around the biome.
To visualise this, there will be a new toggle in some of the visualisations that darken the world, and show foliage height in terms of brightness (brighter grass = taller grass).
As usual, loading old world configurations will have them automatically migrate so that you should only need to tweak things rather than creating worlds from scratch.