selection - Helping users understand that two, and only two options must be selected

I have a drop down checkbox where there are 8 items to check. Two are selected by default.

I am trying to think of an intuitive way to let the user know they have to deselect the existing ones before checking another, and that they have to have exactly two checked (no more, no less).

I think I am happy to use JavaScript to help, perhaps some sort of tool-tip or notification. Any ideas? I'm open to alternative options other than a drop down checkbox too.