QUESTIONDo you have to remove the thin set from previous tile to install porcelain tile. The thin set is adhered to the slab well.
ANSWERANSWER - The answer is it depends... The new installation can't be attached to the underlying substrate any better than what the remaining thin-set is attached to it. So you need to make sure it is well attached if you leave it.
If the thin-set mortar is bonded well to the existing substrate, you can leave it, but you need to grind it down to some degree to get it flat and within the intended plane. After you grind it down you need to vacuum up all the dust and then wash it down to make sure it is clean. Also make sure that water well readily absorb in it to help ensure a good bond between the old thin-set and the new thin-set.
Sometimes even if the original thin-set is bonded well it might make the floor too high at various transitions that can be problematic. In that case you might need to grind the thin-set completely or mostly off.