When editing existing Serial Number records, it is necessary to look up existing serial numbers.  In other words, if you type in a serial number that you know exists because you want to ... say ... change a note, the existing data does not get read into the screen.  It should read in the existing data so that you can edit it.
 
The way it stands now, you have to do a lookup on the serial number in order for the data to be updated on the screen so that you can edit it.