/rehost Instructs a global catalog to drop its copy of a read-only domain partition, then perform a full sync of that partition ...

/rehost Instructs a global catalog to drop its copy of a read-only domain 
     partition, then perform a full sync of that partition from a DC that 
     contains a writable copy of that partition. 

     Targets read-only partitions on Windows 2000 and Windows Server 2003
     global catalogs and read-only partitions of full and read-only Windows
     Server 2008 Global Catalogs.

     REHOST is useful for removing lingering objects in the read-only 
     partitions of Windows 2000 global catalogs which do not support the 
     preferred "repadmin /removelingeringobjects" command, or when the 
     writable domain partition contains only Windows 2000 domain controllers. 

     Cannot be used on any partition for which the target DC hosts a writable 
     copy (schema, configuration or writable domain partition). 

     For example, a GC in root.contoso.com can rehost its read-only copy of 
     child.contoso.com but cannot rehost root.contoso.com.

     A computer executing /rehost may run Windows Vista or Windows Server 2008
     and must have network connectivity to domain controllers specified in the
     "DSA" parameter. Target computers in the "DSA" parameter may be referenced
     by single-label hostname or fully qualified computername (preferred). 
      values are also supported.

     Enterprise administrator credentials are recommended. 

     The global catalog targeted by /rehost must have Windows 2000 Service 
     Pack 4 or later installed. 

     The reference DC must have Windows 2000 Service Pack 4 or later installed 
     and have network connectivity to the GC's targeted by the "DSA" parameter. 

     Time to task is determined by partition size and link speed between the 
     reference and target computers.

     Global catalogs continue to "advertise" and service LDAP requests during 
     the rehost operation even when not authoritative for all partitions in the
     forest. ITADMINS should consider the service impact on native-mode domains
     and application which rely on global catalog consistency. 

     Rehosting a partition is preferred over unchecking and rechecking the gc 
     checkbox because the read-only partitions can be replicated from a 
     deterministic source. 

     Microsoft recommends enabling strict replication consistency before 
     rehosting read-only domain partitions. See "repadmin /regkey". 

     [SYNTAX]

     /rehost DSA   [/application]
     /rehost DSA 
     

     [EXAMPLES]

     repadmin /rehost root-dc01 child.contoso.com child-dc01.child.contoso.com