Uma resposta antiga, mas elegante, que encontrei enquanto procurava resolver o mesmo problema:
bcp Extrair:
bcp YOUR_DATABASE..sysdiagrams out YOUR_DIAGRAM_NAME.bcp -c -T -S SERVERNAME
Importar:
(cd to the directory the diagram is in)
bcp YOUR_DATABASE..sysdiagrams in YOUR_DIAGRAM_NAME.bcp -c -T -S SERVERNAME
