While this is true, i don't personally think Percona tools alone makes these changes 'safe'. You can create scenarios where performance is so degraded that you introduce replication delay or service interruptions (and yes the tools have ways to work around that, but it's a significant challenge to tune this correctly on production), data can be lost during the table switch over, and the complication of managing a product / database with a table in an extended migration is very difficult.

This space is far from done, until we can change data structures on the fly...we're going to find there to be a constant struggle between schema and schema-less and neither side will be right or wrong.

Solution: Quantum states; We suspect every possible scenario is happening already, so we should just migrate our database to the appropriate quantum state, where the data is in the structure we desire. Sounds insane, it probably is.