First of all, don't allow modification of the "unique auto enter serial number that the user does not see" use this field for all relationships. This preserves the links to all related records in other tables.
Make a second field that is initially assigned the value from the hidden serial number field and write scripts that can modify this field by appending additional text to the end of it. Put this field on all layouts where users need to see this information.
Then it's just a matter of creating a script that modifies this second field consistently. The exact steps here can vary depending on where you are when you need to trigger this script.
It would seem that you've pulled up an order in the Orders table and need to issue an additional work order.
A button for "new work order" might look like this:
If [ Not IsEmpty ( WorkOrders:: OrderID ) /* at least one work order exists for this order */]
IF [ Count ( WorkOrders:: OrderID ) = 1 /* there is only one work order */]
Set Variable [$Suffix ; Value: "A" ]
#There are at least to WO's compute the next letter in the alpha series
Set Variable [$Suffix ; Value: Char ( Code ( Right ( Last ( WorkOrders:: OrderID ) ; 1 ) ) + 1 ) ]
Set Variable [ $OrderID ; Orders:: OrderID ]
Go To Layout [Work Orders (Work Orders ) ]
Set Field [WorkOrders:: OrderID ; $OrderID ]
Set Field [WorkOrders:: OrderIDLabel ; $OrderID & $Suffix ]
Go To Layout [original layout]