Vitess supports resharding, in which the number of shards is changed on a live cluster. This can be either splitting one or more shards into smaller pieces, or merging neighboring shards into bigger pieces.
During resharding, the data in the source shards is copied into the destination shards, allowed to catch up on replication, and then compared against the original to ensure data integrity. Then the live serving infrastructure is shifted to the destination shards, and the source shards are deleted.