Tracking a Customer's First Order Date

We have created a custom field "CustomerSince" that each customer in Netsuite will have to track when they became a customer with us. The point of this field is to track when a customer was moved from Prospect to Customer, or their first sales order date.  We initially populated this field with historical data for existing customers when we started using Netsuite. However, we are trying to figure out how to automatically update this field for newer customers that we gain that would not have a value for the field. Without loosing the date already stored for older existing customers