Allow ALTER command to set new rotation INTERVAL while keeping old partitions intact. This involves storing of start timestamp and interval for each partition for pruning to work correctly. The new INTERVAL should be applied to unused partitions as well as to auto-created ones.
When partitions listed explicitly in CREATE TABLE, allow explicit setting of STARTS and INTERVAL per each partition. This information must be reflected in SHOW CREATE (and after ALTER changing the INTERVAL). INFORMATION_SCHEMA should display effective INTERVAL and STARTS per each partition.