You're looking at reconstructing the dress really - you would have to unstitch all the top and use the existing front panels as a pattern, and cut out the shapes from the new fabric, and sew it all together again. If there is a lining on the top (which I guess there is, otherwise the top is see-through) then you have to stitch the new fabric to the lining before reassembling the whole top. If you are not a needlewoman then it's quite a bit of work, and as it's been commercially made then you would have to make sure there are sufficient seam allowances for it to be restitched. To be honest, it's a lot of work and it could work, or it could look bodged - but there's not a lot of fabric in the dress so it would be worth a try. A warning though - sequinned fabrics can be more difficult to stitch by machine, you might have to do it by hand otherwise the sequins can catch in the needle.
Could you not just buy some red sequins of your own, and hand stitch them amongst the black ones?