De-categorize a featured article automatically

Hello again,

The theme I am building shows a featured article.

Thanks to a certain number of conditions on categories, it is withdrawn from the normal column and displayed in a dedicated template.

Now the guys who write ask me a function which will automatically “de-feature” the old post when they choose a new featured one (lest they should forget to do it).

• Is there a plugin to do that ?
• Or what is the best approach ? Create a function ? (I am afraid it will have to test every time someone publishes)
• Any advice welcome.