Purchase Order Tracking

The new feature of Purchase Orders (PO) is great. However, once the PO is converted to a Purchase, there is no record or field to say which PO was converted to make the Purchase. The option to carry over the PO number to the Purchase (as a field or comment) would be extremely useful. This means that you can trace the original PO using the search function. Otherwise all record of the original PO is deleted on conversion, (yes I know this can be turned off). For audit and tracing purposes this would be very useful to do this.