changing server roles in streaming replication
Unforutnatley, you cannot do this without taking a new base backup. This is something being worked on, but at this point repointing the slave is not supported.

There exists a tool called pg_rewind, but there are definite cornercases where that one does not work properly, so it's quite a risky way.

For now, doing a basebackup/rsync is the supported method for this.