Changing existing Labels

Hi Group,

We are in process of revamping our exiting Labels and Sensitivity classes and is planning to introduce new labels as part of revamping and using Microsoft Purview going forward. We do have all the the documents which are being classified as some label on M365(email and SharePoint mainly). Some of them are classified as a manual practice and some with the discovery. 

My question is -

1. How do we change the classification label from old name to the new one ?

2. I also know, that the manual classification will always override the automation one, so if we just introduce the new one or even rename the old ones to new, will this take in place for the manual classification as most of them are manual classification.