Drop-down populated based on whether cell is blank or not

I have a workbook that tracks project status. Basically what I have is two columns that either have dates or not - acutal start and actual complete. I then have a column that has a drop down list with statuses - In Progress, Not Started, Completed, and others. What I want is that by default the status column should be 'Not Started', but then when an Actual Start date is populated, when Actual Complete date is populated, status status changes to In Progress, BUT also will allow the user to select other possibilities from the list if applicable.

I'm new to the board, so I hope that is enough info. I cannot post the sheet due to proprietary info.