Can you explain what you mean be "re-chose C"? Maybe with an example using actual data instead of letters?
I'm guessing you may want a conditional value list here:
Custom Value List? (Forum Tutorial)
http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list (Knowledgebase Article)
When creating a project we put his Status as "In progress" from a value list (Inprogress, cancelled, closed ), if a project is closed it can only be cancelled, so i wanna filter the value list to show only "Cancelled" if Project::Status = "Closed" .
that's the idea
A conditional value list can be set up to do this. See the links in my last post.