We have two custom fields on our Orders page.  If the user selects a value in field A  (a custom pick list) , field B (a custom pick list) should remain unaffected. 

Unless the user selects cheddar in field A.  If this particular value is selected, then the default value in field B should change to super size it .  

Here's the hard part.  Even thought the default value in field B changes to   super size it , we will still need to be able to select another value from pick list in the future.  (As I have the dependency set now, changing field A to cheddar, will change field B to super size it, but all the other options will disappear!  

Can anyone help with this?