how to convert a master cluster to a slave cluster?
This means that I have to wait until all of my files are copied back from the new master the old one, which means a lot of data traffic

When using pg_basebackup, yes.

But, if you're confident that the existing files at the destination are almost identical to the source, rsync may be used. The rsync remote-update protocol is able to identify what parts of files changed to minimize the amount of data to transfer.

See examples of uses on the postgres wiki: Binary Replication Tutorial / Starting Replication with only a Quick Master Restart 

My personal experience with rsync versus pg_basebackup is that rsync is good if the "new master" has not drifted that much from the state when it became master, like a few minutes of transactions on a moderately busy system. Otherwise it still works but pg_basebackup appears to be faster.