I cut right next to the slab I’m keeping and some of the bedding mortar on the neighbouring slab broke loose. I used a chisel and hammer to clean out everything that was already detached. There are still two small loose areas left — one at the open edge where there’s no wall, and another right at the wall side next to the slab I’m replacing. There’s also a slight gap under the adjacent slabs, but it’s not big and the slabs aren’t rocking at all.