We usually do up front design for individual epics. And if we cannot predict what much of the work will be, first we do a spike to sit with the code and any new libraries long enough to guess what the surface area is.
All of these guesses will be wrong of course, but you want to end up with a diagram of how things were, how we would like them to be if time and existing code were no object, and what we decided to do given the limitations of the existing system, and resources (skill and time).
That second diagram informs what you do any time the third fails to match reality. If you fall back to what is, you will likely paint yourself into an architectural corner that will last for years. If you move toward the ideal or at least perpendicular, then there is no backpedaling.
All of these guesses will be wrong of course, but you want to end up with a diagram of how things were, how we would like them to be if time and existing code were no object, and what we decided to do given the limitations of the existing system, and resources (skill and time).
That second diagram informs what you do any time the third fails to match reality. If you fall back to what is, you will likely paint yourself into an architectural corner that will last for years. If you move toward the ideal or at least perpendicular, then there is no backpedaling.