No issue. Just that the introduction to your post got me excited that I was going to learn how to do something I didn't previously think was possible.
You keep the raidz1 pool healthy, create a 5x8TB raidz2 pool with two fake disks, migrate the data, and then only pull the disks from the raidz1 pool when the data successfully migrates. Even if a disk dies during reslivering on the raidz2 pool, you still have 3 of your 4 disks in the raidz1 pool.