Hi, Elisabeth.
You do not have to take the upgrade exam.
You can just take 70-480, 70-486, and 70-487 to earn your MCSD title.
Myself, I prefer not to take upgrade exams because there are too many topics to prepare for.
Remember, after you pass the third exam, the two year recertification timer starts, where you will have to take a recertification exam after two (2) years.
Good luck!
Best wishes, Davin Mickelson