I have not had to replace a previously configured Aquacomputer device, but if the original device still works and appears in Aquasuite, I think you can export all of its settings to an XML file. For my High Flow Next and Leakshield, in the System tab, there are buttons to Export settings to file, Import settings from file, and Reset device to factory defaults. I think you can export the device settings from the old device and then import them into the new device. If you have Overview pages that include items from the old device, I think you will have to select each item and select Change Data Source, then select the same item from the new device. If you have Virtual Sensors set up in the Playground that use data from the old device, I suspect those items will also need to be manually updated. I don't think there is any way to tell Aquasuite to replace Device A with Device B globally.

Aquasuite displays device Serial Numbers in the Updates section in the Aquasuite tab. I think Aquasuite uses the device Serial Numbers to track when a device was first connected. This is used to determine the period of time that you can update Aquasuite for free. If you installed several new devices, this will be based on the expiration date of the device with the longest Aquasuite free updates period. In your case it will be the Leakshield which comes with 18 months of free updates.