is it possible to change shader of terrain details/billboards?

One approach is to use the built in shader source code as a starting point and then make modifications from there. Here's how:

1. Download built in shader sources for your version of unity from the unity archive page. Select the Downloads drop down menu and then choose Built in shaders.

2. Exract the downloaded shaders zip to a folder.

3. Drop the folder into your unity assets.

At this point the downloaded shaders you dropped in will override the built-in shaders because they have the same names as the built-in shaders.

To modify the grass billboard shader in Unity 2017.2.0f3, find the WavingGrassBillboard shader located in the builtin_shaders-2017.2.0f3/DefaultResourcesExtra/TerrainShaders/Details folder.

Here's an example changing the shader to output red instead of the texture color.

If you name your shader the same as the one Unity uses, yours will override it.

For example: "Hidden/TerrainEngine/Details/Vertexlit"

In your case, you'd want the one used for grass.

Source: https://forum.unity3d.com/threads/terrain-detail-mesh-normal-map.176806/

@HowlingMoonSoftware thanks for the reply! (only just had notification of this) And the link :) (hoping i can now make some progress with better vegetation because the stock shader is well limiting with the no shadows and the fact it slices the base off everything creating the sharp edge is frustrating when been careful to not have that on source texture)