I am doing the same thing this week and it was suggested to me to create another pool with a single disk, replicate my existing raidz1 pool onto it. Tear down the raidz1 pool and rebuild it as raidz2. Then replicate it back onto the new pool. That seems simple enough to me, although i am limited by my 6 sata ports and the new pool being 6 drives. I am toying with a temporary sata card or using an external for the temp pool.