Migrating a ZFS pool from RAIDZ1 to RAIDZ2

I recently upgraded my home TrueNAS server and migrated 18 TB of data from a 4-disk RAIDZ1 ZFS pool to a new RAIDZ2 pool. The neat part is that I did it with only three additional 8 TB disks and never transferred my data to external storage. Upgrading from RAIDZ1 to RAIDZ2 without moving data to external storage is tricky because: