You have an Exchange Server 2016 organization. The organization contains two Mailbox servers. You have a
database that is replicated to both servers. The database contains 500 mailboxes.
You add new volumes to the Mailbox servers.
You need to move all of the mailbox data to the new volumes. The solution must meet the following
requirements: Ensure that multiple copies of the mailbox data are available at all times.
Minimize downtime for the mailbox users.
Which three actions should you perform in sequence? To answer, move the appropriate
actions from the list of actions to the answer area and arrange them in the correct order.
Select and Place:
Answer: 
Wrong:you create new database, then you mount it and then you move mailboxes… shortest downtime and replicas are kept intact.
I agree to some extend:
1) Create a new database (empty DB, doesn’t affect current users and numbers of replicas)
2) Create replicated database copies (you need to create replica of that DB, before you move the mailboxes there – to ensure that you have copies of each user before and after the migration from old to new DB)
3) Move the mailboxes (when moving users from old to new DB, you’re moving them to the new volume and as you’ve already created a replica of the new DB, the users will have copies even on the new location)
I agree.