How do I rename relationships in Neo4j?

match (n1)-[old:`Start`]->(n2)
create (n1)-[new:StartDate {propName:old.propName, ...}]->(n2)
delete old

To do the equivalent of a rename, you can create a new one and delete the old one like so:

match (n1)-[old:`Start`]->(n2)
create (n1)-[new:StartDate]->(n2)
delete old

n.b. use backticks like those around `Start` to escape reserved keywords

You are right. You cannot rename an already existing relationship. You'd have to run through all relationships, create the new one in parallel (including all properties) and then remove the old one.

You may also want to consider quoting the reserved word START in your cypher queries with backticks and leave the relationships as they are:

start n=node(0) match n<-[:`START`]-r return count(r)