Batch create relationships
Here's a trick.  Use the API to get relationships with one relationship type and chain an API call to create a relationship with the other relationship type.  You have to explicitly say that the new relationship has "0" as the ID, or else it'll just change the relationships you find.

The following example creates a relationship of type 8 for each relationship of type 7.

CRM.api3('Relationship', 'get', {
  "relationship_type_id": 7,
  "api.Relationship.create":{
    "id":0,
    "relationship_type_id":8,
    "contact_id_a":"$value.contact_id_a",
    "contact_id_b":"$value.contact_id_b",
    "is_active":"$value.is_active"
  }
})


You could add parameters to the Relationship/Get call to just pick certain relationships.  Theoretically, you could also chain all of this to another API call, perhaps to get the members of a group.