It is possible to replace debug nets connected to an ILA core in a placed and routed design checkpoint. You can do this by using the Engineering Change Order (ECO) flow. This is an advanced design flow used for designs that are nearing completion, where you need to swap nets connected to an existing ILA probe port. For information on using the ECO flow to modify nets on existing ILA cores, see this link in the Vivado Design Suite User Guide: Implementation (UG904).