Rotation vs rekeying

What's the difference between key rotation and rekeying?

Rotation is switching which key gets used for new writes going forward — it doesn't touch anything already written. Rekeying is the separate background process that goes back and moves existing records off an old key onto the new one, which is what eventually lets the old key be deleted. Rotation is quick and low-risk; rekeying is the slower, higher-effort part. See Key Rotation, Rekeying: Encryption, and Rekeying: HMACs.