Relaying patio

Hi all,

I’m looking for some help and advice as I have decided to relay my patio. The current slabs are the standard buff concrete type about 40mm thick and I want to replace with 20mm porcelain. I have lifted one of the existing slabs and found a full mortar bed which seems in decent condition.

my question is can I put a new full mortar bed on top of the existing one and then lay my slabs directly onto that? Or do I need to completely remove the old mortar bed to effectively start from scratch?

due to the difference in thickness of slabs It would leave room for a new 20mm mortar bed on top of the old while keeping the patio at the same level.