I'm jumping in on this topic as I'm in the middle of the same process but taking a different approach. I didn't want to remove my one parity drive and then rebuild a new parity drive as that would put my array at risk. I followed this process, please tell me if there is a problem.
I currently have 1 4 TB parity drive and my array is filling up. I leveraged the black Friday deals and bought two 20 TB drives and have an extra 14 TB drive as well. I added the 14 TB drive to the data array and one 20 TB drive as "Parity 2". My plan is to restart the system and let Parity 2 build parity. Once that is done, I will add the 2nd spare 20 TB drive to the data array (now that I have a 20 TB Parity 2 drive) and move the 14 TB drive that was formerly Parity 1 to the data array. Restart the system and the array will come back on line with 1 parity drive in the Parity 2 position.
Any issues with that approach? Thanks for the help!!