How can we change vray proxy materials?

[arnau]

Hello,

I have done a forest with different proxies trees.
To create the trees I followed the instructions from this link below:

http://help.chaosgroup.com/vray/help/sketchup/150PB/tutorials_2d_proxies.htm

Now I would like to change some settings from the trees textures to effect on the Alpha channel.

How could I change vray proxy textures once the proxy is done?

(I want to use the alpha channel for post production in some program like Photoshop)

Thank's in advance!