InfoPath Dev

I have a list with InfoPath 2010 and Designer 2013 running on it. Within the form, I have a dropdown control called Status, which by default is blank, and has the following values: Requested, Forwarded and Received. What I'm trying to accomplish is if a user changes the Status to any of the dropdown values, then the user cannot change it back to blank. Any assistance would be appreciated.

This is a list form? I think I'd probably just add a helper field, true/false, that is set to true once the status has been selected, and then add a custom validation rule on the status field that if the field is blank, and the helper field is true, there is a validation error. Does that make sense?

Thanks it works!! Not sure why I drew blank but this is exactly the push I needed.