You approach would work, but at the end you would have parity2 but no parity1 drive. Some people prefer to not be in that situation..
Another option is to replace the 8TB parity drive with the 16TB one and let Unraid build parity on that. Keep the old parity drive intact until that finishes just in case something goes wrong. When you have rebuilt parity on the 16TB drive follow the standard process for adding a new drive to the array to add the old 8TB drive as a data drive.