My question is also same that others asked before.
I found that once you set the Icon for the Application we can\'t change the Application
Before iOS 8 i tried one way which give me positive result at minimum once:
set icon.png as link to ../Documents/icon.png
But you need to force ask Springboard to redownload icons for installed applications (cause Springboard cache them)
Since iOS 8 this way does not work cause Documents folder was moved to dynamical path.
So you should find way to update readonly icon.png from bundle.
For apps installed to /Applications/ (through Cydia) - it's possible to do.
For apps installed via adhoc - There is no way to update readonly icon.png (as i know)