I don't think Cypher lets you do anything fancy besides SET for MERGE's "ON CREATE" and "ON MATCH".
One approach might be to do this in separate Cypher queries.
However, this might also do the trick:
MATCH (nodeA {id: <nodeA_id>})-[r:OWNS]->(nodeB), (nodeC {id: <nodeC_id>})
WHERE (nodeB) <> (nodeC)
MERGE (nodeA)-[:OWNS]->(nodeC)
DELETE r
And, if at all possible, do try to use labels for identifying your nodes, e.g. NodeA:User. This will speed things up greatly.
Let us know if this works for you.
HTH