May I break in? Just did some 25min testing. One tree (and grass) growing and changing the leaves texture from bigger-leaved to smaller-leaved over time, through merge shader with a world scale transition from black to white as controller. I can't really see if that works, perhaps the differences in leaves are too small.
I also tried changing the opacity over time, but that had a strange blackening effect on the leaves. I still don't know if world scale opacity works by now. It didn't a while ago. A way around this is to use a black opacity default shader and a normal, merged and controlled as said.