After you save a value into a Subject field on a case, it's very difficult to change the value to something else. There is no icon for opening a dropdown menu of the values and clicking on the field will open a modal dialog window of the Subject record itself. Only after going through this mistaken click is the user able to reveal the icon for opening and navigating the subject tree values and updating the field. The only other option seems to be to click a couple of pixels wide area just at the beginning of the Subject field, without hitting any of the texts from the value that is currently populated for the field.

Please provide a feature similar to the standard lookup fields for opening the subject tree value selection menu for a non-empty Subject field.