Adding a mailbox role to an existing exchange 2013 CAS server

Hi,

We've recently migrated our on-premise Exchange to Office 365 but we keep management of users in our on-premise Active directory and dirsynced the attribute changes to Office 365. As per Microsoft recommendation, an Exchange server with CAS and Mailbox role should still be on-premise to manage the Exchange attributes. I have two Exchange 2013 servers, one CAS and one Mailbox, I wanted to decommission the Mailbox role server so that I can reuse the storage.

Is it possible to uninstall the Mailbox role and then add the role in the existing Exchange CAS server?

Or, I'll just uninstall Exchange on the 2 servers and setup a new Exchange server with both CAS and Mailbox role?