Well, I may be getting turned around, but you never populated matches.SerialList_c with anything, so the column doesn’t exist yet (wasn’t created/initialized with a preceding row-update). That’s probably why you’re getting undefined.
Still no dice.
Just try this simple expression:
{UD100.SerialList_c} + ', ' + {matches.SerialNo_SerialNumber}
The worst that can happen is getting a preceding comma if SerialList_c is empty.
Ok, that results in no error, but like I was seeing with @Randy’s example, it’s writing blank for my UD field, while also not concatenating through each iteration. And if I change UD100 to matches, again get the undefined even though “SerialList_c” exists in the dataview.
I’ve got an Epicor technical consultant contact and some contracted hours remaining with them, so I think at this point I’m going to reach out to them for help.
Really appreciate all the help throwing darts today!
Well, that expression wasn’t trying to concatenate over the iteration… I just wanted to get it to concatenate period. Which it did for me.
Edit: above, not a true statement 
I was just testing and the row-update WILL accept using the current field value in the expression. As a test I did:
'{TransView.TextBox3}' + ' ! ' + '{TransView.TextBox}' + ', ' + '{TransView.TextBox2}'
I preloaded my TextBox3 value:
Clicked the button:
Clicked again:
So… iterative concatenation should work. Let me review some of your setup. i think we’ve made so many changes that we may be missing something simple.
Can you change the row-update back to UD100 and snip/paste the row-update event in the debugger? I’m curious what its setting the value to each time, since it is not concatenating for you.
Gentlemen, I think we have success!
Expression: '{UD100.SerialList_c}' + ', ' + '{matches.SerialNo_SerialNumber}'
Just need to get that leading comma out of there.
Edit: updating the expression to this clears that up (while leaving an ending comma that I can live with)
'{UD100.SerialList_c}' + ' ' + '{matches.SerialNo_SerialNumber}' + ','
Noice!
We tried to do the check in the expression, but that may not be doable.
You could throw a condition before the row-update.
'{UD100.SerialList_c}' !== '' && '{UD100.SerialList_c}' !== 'undefined'
If true, do your existing row update:
If false, row update UD100.SerialList_c to the exression '{matches.SerialNo_SerialNumber}'
It will check that condition during each iteration, but should only go down the false leg on the first pass. After that SerialList_c will not be empty and will go down the true leg each time.