I think the Wagg Bunny Brunch is a mix isn't it? These types of food are usually more fattening than a high fibre pellet so that coupled with the facts that she's had lots of it and little or no hay has probably contributed to her weight gain.
As I said before, all food changes need to be made very gradually. I would just start reducing the Bunny Brunch little by little each day until she's getting maybe a flat desertspoonful a day. At the same time I would try to get her interested in some hay (with the reduction of mix she should naturally turn to the hay to fill up) - ordinary meadow hay is fine with perhaps some timothy as a treat, put it in hay racks, in boxes, in her litter tray - they like to poop and munch at the same time!, stuffed in to toilet roll tubes anything to keep her interested - you could scatter her ration of bunny brunch amongst it to encourage her to forage and eat the hay too.
You could put her bunny brunch ration in the treat ball.
Its up to you really if you decide to change to the high fibre pellets (Science Selective are a popular choice, Excell is widely available too but some people find their buns have sticky poo problems with that one) as she's older she may be set in her ways and so long as she only gets a very limited amount of mix and eats mostly hay, I shouldn't think it will make a lot of difference really.