I have this same issue on a flight back from ARN-JFK-RDU next month. I've already spent 3+ hours on the phone with res just trying to get old GUCs from a cancelled itin re-credited to my account and then apply them to this one (there were other issues the first agent caused by messing this up).

It's confusing because when I booked it, it said on DL.com "D1 upgrade available with GUC" but that's misleading (at best, false advertising at worst) because PE is NOT available so I can't book it, and therefore can't be upgraded to D1, because main is the only thing available.

The additional issue for my case is the JFK-RDU segment which cleared immediately into F upon adding the GUC...so now if I don't get an upgrade at the gate, I've potentially wasted a GUC on JFK-RDU regional jet. If I don't get cleared from ARN (and it looks like D1 is pretty much wide open on my flight so far) I'll immediately be on the phone with DL to get my GUC removed. From part experience I've found upgrade handling at European outstations to be wildly variable. So I reckon I'll keep my fingers crossed.