@Shawna_Wright -
I am not aware of anyway to make it go back to the default state that exists before a grade has been entered just by changing what has been entered in the grade box. Once something has been entered, there is probably a state variable(just my phrasing, I do not know what the correct name should be) that indicates that a grade has been entered and when you delete the grade that state variable stays set.
Having said all of that, there is probably a way through API's or javascripts to change that state variable to the default value. Much more involved solution for the regular user. I do not have the answer for what I mentioned about API's or javascripts.