Replacing a filling is much like adding a new filling, so in general the answer to this question is no; you will not experience much discomfort during the procedure (and possibly none at all).
We will use local anaesthesia to numb the area before starting any work. You may feel some tenderness or discomfort in the area for a few days after the procedure, but this will pass.
You can manage any discomfort during recovery with over-the-counter pain medication, like ibuprofen.