How do I toggle between a Vertical and a Horizontal split in vimdiff?

The following command will change a vertical split into a horizontal split:

ctrl+w then J

To change back to a vertical split use either:

ctrl+w H or ctrl+w L

For more information about moving windows:

:h window-moving
:h ctrl-w_J
:h ctrl-w_K
:h ctrl-w_H
:h ctrl-w_L