I wanted to rearrange disks on my tower, and looked up tips on the forums. Similar threads from 2017 (referring to v6.2.x) have advised that a new configuration may be needed, and parity 2 may need to be rebuilt.
I rearranged disks and booted up to v. 6.4.0, to see that the array had already started and disks were detected automatically. The disk identifiers (sda, sdb, etc) have changed, but it would seem that unraid now pairs the parity and data disks to their serial numbers automatically, without having to create a new config or rebuilding Parity 2.
Just wanted to share this with fellow users for future reference.