Help! Christmas Cake ingredient quantities

I messed up & put 1kg of soft dark brown sugar into my cree Christmas cake mix, it was measured out for Christmas puddings! My cake mix needed 1.5kg of demerara. Should I make the 1.5kg cake amount(6 x recipe) with 500g of sugar? Will this affect the cake? I can't use the normal cake recipe as it will be too sweet? Help!
Emma