change the retention type on my VBM365

Hello,

I'm trying to change the retention type on my VBM365 backups from Item level to snapshot, but it doesn't give me the option to change it if I select the same folder on the S3 bucket. Will I be required to initiate a new full backup?

A second question: I want to understand how Item level retention works, as it's not clear to me. I have a 5-year retention on Exchange, how does it work in the background?

Thank you