Setting the alpha for already drawn line

Hi,

I am drawing lines using Shape object, at each frame the line is being drawn. However at particular even, lets say after frame 400. I have to set the alpha of already drew lines (lines that were drawn before frame 400) to a particular value. How can i?

I have read the documentation of graphics lineStyle and it says it’ll set for the lines which will be drawn, not for the ones which are already drawn.

it’s REALY URGENT