Changing skins? If you want change VIEW(1st view) MODEL of grenade, you need following this rules: Valve Model, Valve Texture = No.
Valve Model, Custom Texture = No.
Custom Model, Valve Texture = No.
Custom Model, Custom Texture = Appears to be OK currently. (Copied from ZP for CS:GO)
If you want 3rd model of grenade, you can just set model.
On client side you can change ONLY via server side