For consistency, the icon in your taskbar is the icon on your form (so it makes sense to the user).
If you want to do this, the only way I can think to do is some "slight of hand".
- Change the "show in taskbar" property to false
- Create a new form and force it to stay minimized
- change your icons as you want on your new form
a better choice might be to do this interaction with the "notify icon" and hide this forms taskbar button.