I currently have 6 disks in my array. I have an 8tb parity drive and 5 4tb data drives. I am downsizing and want to remove the 8tb parity, and replace it with one of the existing empty 4tb drives. I then want to take the 8tb and another empty 4tb drive out of the system to use for a different build. What would be the safest way to do this?

Since there's no files involved on any of the drives, Tools - New Config, and reassign the drives as you wish.  Parity will rebuild.  Just double check that those 4TB drives are empty.

I'd recommend a parity check before doing the new config, and reviewing the smart reports of the drives for problems. Once you do the new config, your ability to recover a failed disk (one of the 4T drives containing data) is lost until the parity rebuild completes.