How to retrieve old messages from DB[In case of persistent chats] when private keys are to be changed?

The project is good, but shouldn't the private key change with time. If it changes based on time period it might add additional layer of protection. My doubt is when the key changes how to decrypt all old messages from the database. 
Since this project does not persist the messages it's fine.
Any solution will be immensely valuable to me.
Thank You!