Custom Lists - Updating from active Yes to active no appears to be same as deleting?

Hi

I have a custom field that has grown over time, to make drop down more user friendly I updated some of these to active no. My expectation was that these would no longer appear in the drop down but the history would be retained.

However this action appears to be the same as selecting the delete option. I have now lost some of my incident history and have the added annoyance of missing mandatory fields? 

Is there a way of making these values active again?