If you have tutorials enabled there is one on 'Dragon Morphing' (the voice-over just says 'dragon morph button', since it can be re-assigned, but the subtitles include the default hotkeys in gold text). The symbol in the top middle of the bottom UI allowed you to transform in earlier betas, but the lack of a mouse cursor in dragon mode meant that if you didn't know the hotkey the only way back to RTS mode was to kill your dragon (I don't know if that was why that was removed, though).
