BPM on SalesOrderUpdate, want to update OrderDtl field when multiple criteria in OrderHed and OrderDtl met

Because we allow the flexibility to our CSRs to enter sales order in any order they want to (entering the ShipTo location first or after adding lines), I changed how I handled this task. I created Two Post-Processing Method Directives. One is on SalesOrder.ChangePartNumMaster and the other is on SalesOrder.ChangeShipToID. The one on ChangePartNumMaster I was able to update the comment box with the message for the CSR and provided a popup to notify the CSR that had occurred. If the CSR changes the ShipTo after the lines are added, I provide a PopUp Message, but I could not figure out how to update the correct lines on the order. Does anyone know how to cycle through the lines, find the lines that have matching part numbers, then not erase any possible existing text in the Lines-Comments-Pick-List/Job box but add a note to it?

Here is what my BPM looks like:

Because this update is occurring on the SalesOrder Header, no data exists on the ds.OrderDtl table.
How can I determine which lines match the part number I am looking for?
How can I copy and add to the existing comments on all the lines that match the part number I am looking for?