Hi @jb905 
Welcome to the Canvas Community. I assume that you are a student, since you are specifically asking about the cool "What-if" function in your view of Grades. You ask an interesting question, since I didn't realize that students could not test an EX grade, but upon my own testing you are absolutely correct....you can't! However, there is likely a good reason for this: so far as the final grade for a course is concerned, leaving an assignment with a dash grade ( - ) is essentially the same as typing in EX for "excused," since both count neither for nor against a final grade. On the other hand, if a faculty member has already input a grade for an assignment, all you can do is test another grade....you cannot try to ignore it or type in EX. So, insofar as a FUTURE (ungraded) assignment is concerned, just leaving it blank with a dash is effectively the same as typing in EX. But for a past/already graded assignment, indeed it is not an option.
Hope this helps a bit, Jordan.
