If you want to keep the old backup.. then the easiest way is to get a USB drive.. plug it into the computer, and use that as TM target.
You can also plug the drive into the TC and enjoy slow backups.. USB drive on TC runs at less than half the speed as local.
Finally you can use the USB to archive off the old backup.. erase the TC and start over.
If you don't care about the old backup then just erase it.
You might also be able to connect the new backup to the old one.