Two shaders (B) Fresh Leaf & (A) Wilted Leaf are plugging into a (C) layered texture (Then the final output to (D)a surface shader). Then an animatable transparency ramp map is connected to the (B) fresh leaf shader, therefore (D) the leaf shader transforms into wilt leaf. (Revealing colour & Bump information from the (A) wilted leaf shader.) (Fig. 01)
Fresh leaf shader (B)
Pretty standard shading network as you can see. Lets get into the concept of ($) and (*) down below. (Fig.02)
Fresh leaf shader
Leaf_crack01 is a image file map (It is a painted map in Photoshop, the white areas are where it will wilt) connected to a ramp shader Wilt_Trans01.The black part in Wilt_Trans01 map is the part that has crumbled away. This map has to be plugged into both shaders (A),(B). (Fig.03)
Two files connected network
Out colour R to U,V Co-ord. As long as they are black & white map you can use others such as Out colour G,B.
By connecting the file texture to the ramp, you have a set up for animating values. If you want harsh edges between values, you can clamp the position of ramp like the Fig.05 images shows .