It is possible, but you will lose a lot of fluid when detaching hoses and such. So have some extra coolant + water on hand to refill when you are finished. Also, if you don't drain the system ahead of time, the coolant sprays everywhere. So if you are a neat freak, drain the system first.
So, you don't NEED to drain the system first, but it is helpful. And draining the system only takes a few minutes.
