I prefer the online approach regardless of downtime. Breaking the migration into incremental and backwards compatible steps is less stressful with smaller tail

I prefer the online approach regardless of downtime. Breaking the migration into incremental and backwards compatible steps is less stressful with smaller tail risk exposure than trying to do it as a single state transition. I think the parallel change approach seems more complicated at first but ends up taking less work and less effort in the long run because of increased safety and incrementalism.