How to remap tmux split pane keys?
I needed to reload (perhaps 'apply' is more appropriate) my configuration file.
I had restarted my mac and thought this would have done it but I still needed to do:

$ tmux source-file ~/.tmux.conf


Note: type this exactly as shown.  source-file is literally what you type (or copy-paste) it is NOT something you replace with your own value).