1.The error means the import subnet already authorized in domain, you need to unauthorized and stop old DHCP server.
2.Just use netsh to merging DHCP scope following
3.Merge into one scope? Not exist this tool. Just could export and import
DHCP address list, but it doesn't make sense.
4.To move DHCP from Windows 2003 to Windows 2008, suggest you remove scope option first and then follow
If there are more inquiries on this issue, please feel free to let us know.
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