How can I overwrite the built-in grass shader?

I am trying to overwrite the built-in terrain grass shader so that I can edit it. I have imported the built-in grass shader to work off of, but my terrain seems to not be using the imported shader. No matter what I do with the shader, my grass still looks the same. I have no errors or warnings, and I have not renamed or changed the built-in shader that I downloaded.

(I am using Unity 5.3.1)

Thank you for your time, I would greatly appreciate any input.

This is the shader I downloaded (I am aware that non-billboarded grass uses a different shader, and my grass is set to billboard):

 Shader "Hidden/TerrainEngine/Details/BillboardWavingDoublePass" {
     Properties {
         _WavingTint ("Fade Color", Color) = (.7,.6,.5, 0)
         _MainTex ("Base (RGB) Alpha (A)", 2D) = "white" {}
         _WaveAndDistance ("Wave and distance", Vector) = (12, 3.6, 1, 1)
         _Cutoff ("Cutoff", float) = 0.5
     }
     
 CGINCLUDE
 #include "UnityCG.cginc"
 #include "TerrainEngine.cginc"
 
 struct v2f {
     float4 pos : SV_POSITION;
     fixed4 color : COLOR;
     float4 uv : TEXCOORD0;
 };
 v2f BillboardVert (appdata_full v) {
     v2f o;
     WavingGrassBillboardVert (v);
     o.color = v.color;
     
     o.color.rgb *= ShadeVertexLights (v.vertex, v.normal);
         
     o.pos = mul (UNITY_MATRIX_MVP, v.vertex);    
     o.uv = v.texcoord;
     return o;
 }
 ENDCG
 
     SubShader {
         Tags {
             "Queue" = "Geometry+200"
             "IgnoreProjector"="True"
             "RenderType"="GrassBillboard"
             "DisableBatching"="True"
         }
         Cull Off
         LOD 200
         ColorMask RGB
                 
 CGPROGRAM
 #pragma surface surf Lambert vertex:WavingGrassBillboardVert addshadow exclude_path:deferred
             
 sampler2D _MainTex;
 fixed _Cutoff;
 
 struct Input {
     float2 uv_MainTex;
     fixed4 color : COLOR;
 };
 
 void surf (Input IN, inout SurfaceOutput o) {
     fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * IN.color;
     o.Albedo = c.rgb;
     o.Alpha = c.a;
     clip (o.Alpha - _Cutoff);
     o.Alpha *= IN.color.a;
 }
 
 ENDCG            
     }
 
     Fallback Off
 }