If I apply a SWU and it clears (econ to business), and then I want to make a voluntary change (say, leaving a day earlier or later) what fare class is required on the new flight to preserve my upgrade?

I understand that for involuntary changes they will try very hard to preserve it. But for voluntary I assume that there has to be room in particular class... does that require "upgrade C" on the new flight? Or is it something else?