Is there another colleague who has expressed an interest in swapping the Sunday night with you?? If so, and you do the same hours then the manager can just go whistle in the wind...they are being obstructive and unreasonable.
If you would rather just drop the Sunday shift, then you fill in the opt out form, giving the four weeks notice...you do not have to take on another shift instead, regardless of what the manager may have you believe, making out they're doing you a favour.
Not only is it your choice to opt out of Sunday working, it is your legal right...again, regardless of what the manager may think or say. The opt out forms should be readily available for you instore, either from the wage clerk or admin assistant, or your night manager..any stalling...greivance.