This manual page does not mention what happens if you have already created users in the standalone database, before converting it to a replica set. I have sought for a solution for a long time, because I was not able to issue 'rs.initiate()', with the error "not authorized to execute command on admin".

I think it is valuable to explain that, if you have existing users, one of them should be granted the 'clusterManager' role: http://docs.mongodb.org/v2.6/reference/built-in-roles/
Then, this user can be used to log in to the Mongo shell and issue 'rs.initiate()'