CHange VideoBackground Shader at Runtime

Hi, As the title said, I can't get out of this. I tried

"VuforiaConfiguration.Instance.VideoBackground.VideoBackgroundShader = myShader  "

as it is not read only but it doesn't change the shader in the configuration asset. I also try to add

"VideoBackgroundManager.Instance.OnVideoBackgroundConfigChanged();"

but nothing happen at all. Can't find it in the API. Any ideas ? There's a set fonction for the Matte shader but not for the video shader ?  

THanks in advance !