You may find this just does not work.. Especially with Catalina as per your profile.
Apple's instructions being a long way out of date.. and success with this sort of moving backups failing badly since Sierra where there was a major security upgrade.
In practice it is NOT a great idea anyway. If you wish to keep your old backups.. we suggest using the archive function in the airport utility to a USB drive plugged into the TC.. and if the need to recover files comes up use the USB drive directly on the computer. Or if you are not going to use the TC anymore you can put the whole thing in storage for a year or so until you build up sufficient history that your need to go backwards is exceedingly unlikely.. and from that point you can erase the TC and sell it off.