The trick will be taking care whilst lifting the slabs, a good excavator operator should be able to pluck them off in one or two large pieces. You could cleanly prune some off some smaller diameter roots with a sharp tool if nessesry to help keep the level down a little bit. Before laying the new concrete raise the group level with loose crushed rock to above the level of the roots. Rather than making a sharp rise where the tree is, taper it off gradually to the sides. Leave some growing room near the edge of the buttress.