There are three F2's in the OOB's the CS is still grey but the other two are now
to change back to the default Icon assigned in the OOB you need to change the column that shows Changed ? Yes/ No in Scenhack to No then save in Scenhack and that will reset the Icon to the OOB default....you need to reload your scenario to see the changes if it's running when you do that
I'm not 100% sure that doesn't also reset any other editing you may have done to a unit though. If you simply change the Icon number back without changing the Changed flag it will treat that icon number as the new "custom" one becasue the changed flag tell it it's been changed