After reviewing this guide
Split domain with Exchange 2003 as Primary - Zimbra :: Wiki

I noticed that split domain works in a way that Exchange being the primary mail host (that's the way it should be for my scenario), and the new ZCS being the secondary server, Exchange will forward the e-mails towards the ZCS only if the mailbox doesn't exists in the Exchange server. Is this correct? What am I missing?

The scenario I would like to have is one in which:
1) Exchange server is primary and ZCS is secondary
2) Provision all user accounts into ZCS
3) Configure mail forwarding from Exchange towards Zimbra
4) Start data migration (please, any example script that you may know will be much appreciated as rules, tasks, mails, contacts, distribution lists, public folders, etc should be migrated. At this point, users will still should be using Exchange while the new messages are being forwarded as for configuration in step 3.
5) After migration is complete, change proxy/routing to point to new ZCS.
6) Shutdown Exchange

The LDAP server is Active Directory and will continue to be as we don't want to change user passwords.

Is this possible to do? Please, could you please tell me how or point me to some document that says so? If this is not possible, what is the closest workflow to achieve this?

Thanks a lot!!!