AFAIK the block size is a pool wide setting. So you can't even mix and match block sizes, let alone incrementally upgrade it. And what's more, you can't even change the pool wide setting - so the only solution is to create a new pool and rsync your data up (which is just horrible!)

However I've only done minimal investigation into this issue so if you do find a safe way to upgrade the block size then I would love to know (I'm in a similar situation to yourself in that regard)