It gets pretty confusing after step 2. Maybe I missed something in your previous posts (haven't reread them), but I can't imagine why you want to get Unassigned Devices and parity rebuilds involved.
Probably don't even need step 1. After step 2, you have moved all data off disk3, so you can just stop the array, change its format, start the array to reformat. Parity remains valid, disk3 is now XFS and empty, ready for you to move data onto from any other disks you want to convert.
Not sure what is getting you confused here and making you come up with this scenario. Basically you just need to move all the files from a disk so you can reformat it to another filesystem. Formatting a disk while it's in the array updates parity at the same time. Format is simply a write operation, it writes an empty filesystem. unRAID treats this write just like any other, by updating parity.
OK, I just went back and read this post you made on another thread
At the point where you said you replaced a drive, you seemed to think unRAID decided to format the replacement to XFS. It didn't format anything. It rebuilt the smaller drive (which had just been formatted XFS) to the larger one.