Help with Shared Mailboxes

dovecot version = 2.2..27 I have a running debian server ( jessie-backports ) supporting email for three different domains. It is working fine. My boss has asked me to look into using Shared folders/mailboxes - for instance, he wants selected users of Domain-A to be able to see the shared Inbox of the user "support" who is also in Domain-A.  Sorry if my questions are basic, but here goes:-

1. Can I simply change the relevant dovecot configuration files and introduce Shared mailboxes into an already running system?  Very basically, add the imap-acl plugin, the shared namespace, the dict file and restart dovecot?
2. If I can do this, would it wipe out the tens of thousands of existing emails I have on the system now?   i.e. Does Sharing have to be introduced when new email server is being brought up? Can it be added on-the-fly to my working server? Would the emails in /var/mail/vmail/Domain-A/alan/mail seemlessly populate /var/mail/vmail/Domain-A/Shared/alan/mail after making such changes?
3. If adding the Sharing to my running system is possible, can I add it on only one of my three domains? I have a test domain with one user.   My principle domain ( Domain-A ) has many users, and as I stated previously, very very many stored emails. Emails I absolutely cannot afford to lose. Any insights are appreciated. Alan Stevens

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