I didn't find the documentation about this before trying to find a solution to the upload problem. Having come across the documentation I tried it in case I was missing out on a simpler way; like you, I found it didn't work. I'm sure it does work in the right circumstances but it's quite easy to trigger bugs in some of these exensions - perhaps this is the problem.
Assuming your update form has a hidden variable containing the filename currently stored in the database, simply setting the DefaultFileName seems to work, eg instead of:
'DefaultFileName' => "".$WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"] ."",
(as suggested in the documentation)
'DefaultFileName' => "".((isset($_POST["imagefile"]))?$_POST["imagefile"]:"") ."",
where imagefile is the hidden filename
<?php echo ((isset($_POST["imagefile"]))?$_POST["imagefile"]:""); ?>
in the Default file field using the extension interface.
I don't find these extensions very robust when compared to the Interakt extensions (which I recall you use) - often the code needs tidying up for stray tags etc. and bugs triggered by things such as undescores in fieldnames. Also, for creating a finished form, including validations, sorting, navigation, image upload and debugging etc. the time taken WA:Interakt is something link hours:minutes. Having said that, you are more in control of the finished result. I think some of the extensions are very good and others could be very good. My concern is that they could become worse rather than better, if for example greater integration is made between CSS forms and DataAssist. Hopefully I will proved wrong.