In our experience, it's much easier to keep the existing single server as the LDAP master and add new mailbox servers for your additional mailbox needs.
You can always move the mailboxes on the existing single server to one of the new mailbox servers if you want no mailboxes on this LDAP Master.
For that many mailboxes, you will probably want to add one or more dedicated MTA servers too, each of which should be configured as an LDAP replica.
There is a multi-server installation guide which has the details on how to expand an existing single-server installation into a multi-server installation.
Hope that helps,
Mark
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___________________________________ L. Mark Stone, CIO "Uptime. All the time."
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