Is it possible to change the dock icon of an application only?
A workaround is to make an alias of the app, drag the alias to Dock and then change the icon for the alias ie. not for the actual app. Downside is that you get a small alias–arrow added to the lower–left corner to the icon.


Make an alias
Drag the alias to the Dock
Change the icon like you would for any Finder item.  

In the following screenshot we have copied the icon of Chess app and pasted it to the newly–made alias of Automator. (NB, make sure you first copy the icon and then paste it; the screenshot accidentally is showing the opposite way.)