Update
Apple has finally followed my lead and this is no longer necessary as of macOS Catalina.
Original Post
More "getting stuff ready" info so i don't forget. To change the default shell in OSX the venerable chsh program works just fine.
chsh -s /path/to/zsh
Now here's something that's also good to remember. If you've started a terminal session in something like tmux in your old shell, and then use chsh to change your new shell the value of the $SHELL variable will still be the old shell. You'll need to exit tmux to get things working absolutely properly. You're welcome. Now your zsh reload() function will actually work properly again.