Bulk re assigment of relationships
More of a thought than an answer (and really off the top of my head) - try using the Merge Contacts option, have the person with existing Relationships on the left, newbie on the right, untick all boxes other than Relationships, Merge. 

Revisit your 'deleted' contact and undelete them - does it restore the relationships to them (I don't think so). Does this achieve what you need? Pls report back.