How to make it clear that a value is being changed using keynote
Use the Inspector tool to add build-in and build-out transition to your texts and cross.  

You'll have 4 build order steps :  


cross fade in 
cross disappear
text "20%" disappear
text "40%" appear


Play with duration and 'start build' settings to obtain desired effect.
See http://www.youtube.com/watch?v=1X-5AizWIJE for a screencast ;)