Cash: Have you checked to see if changing the desert biome breaks any existing uses?
I've taken a look around at the existing builds in Dorne and by and large they only use Mushroom Island for grass (since Desert currently produces a darker green than Plains). I haven't seen it used deliberately for grass at all, just for making water brighter, which won't be affected). The only place it's used consistently is along the Worldpainter generated Greenblood, sprinkled along the banks.
Considering that the current Greenblood looks pretty gross with that dark wet sand anyway I think the lighter tone of the Desert 2.0 grass is an improvement.
I'll continue searching with the desert highlighted red to make sure Highgarden's not desert or something, in the meantime we could have a think about the best colour tones to go with.