Unique meta values in articles

Hi all,

I have a custom meta field in my default post type, where I can select a value from a drop down menu (NULL, 1, 2 and 3).
Consider I have 4 posts each one having the corresponding value and when adding a new article, giving the value of 1, I’d like to remove that value from the older article, upon save, thus having a single article with any given value.
I know that getting the latest article for each value would be an option, but I want to change the older article’s value for other reasons.

Thanks in advance,