I've seen a few where the bolt head sieze in James and have had to be hit with the impact gun to rattle them loose.
As Bigash says above, the disc needs removing from the stub axle to access the bolt heads. Then just hit the old one out and replace it with a new one. It is worth noting that x3 of the bolts are one size and x1 is slightly smaller.
You have to remove the rear disk from the stub axle to get access to the head of the bolts and then they are just normal bolts. Cant understand how you’ve managed to get them undone without taking the disc off tbf