Xandrios That's interesting. Now that I checked, both MAD and BCN can both be purchased as a single-city pass, so perhaps the two dutch cities will follow soon? But I agree, quite expensive. MAD ist just 1€ cheaper than "Spain", but BCN is 11€ cheaper (per ticket), so perhaps if they "split" the dutch cities as well and you just need AMS, then it could get a couple of €€ cheaper.. (however if it is just a matter of 1-2€ and you've made up your mind about buying the 0-day pass, I would always choose the multi-city pass, to be able to double-down on destinations, in case one of them is full)

DUS taxes are btw also over 45 Euros, so also quite a lot. EIN is like 41€, so cheaper than DUS and probably also easier for you to get to (?).

I haven't closely monitored availability on the AMS-ATH route, but even in 2 days from today there seem to be even U/T fares available, whereas the DUS-ATH flights are in part completely booked (if not over-booked!).
What I mean is, I see absolutely no point in purchasing a 0-day pass for DUS (since paying more for it and then not being able to redeem due to full flights will be a huge let-down), but perhaps it could be "viable" for AMS (you can certainly assess this better than non-AMS passengers can). And I say "for AMS", even though it's still only available as AMS/EIN, because EIN has like 2 flights per week, so a 0-day pass for that is nearly completely pointless!

I don't quite understand why you'd have to wait until OLCI to request an upgrade with vouchers. If you do book over 48h prior to departure, just apply for an upgrade immediately. Should I class open up at OLCI and your voucher be high enough up the waiting list, it should also clear (or am I missing any "tricks" regarding this process there?). Moreover I don't understand your concern about changing flights after having successfully upgraded. If you're using the 0-day pass, then, by the time you redeem your pass, your flight will be a couple of days, down to a couple of hours away, so I'd say you either definitely know you're flying or you're not booking in the first place! Having used a 30-day pass I've always refrained from placing an upgrade request until I was absolutely certain the booked flight was to be taken (it's actually a good thing you can still re-book after you've entered those final 30 days prior to the flight, or that you can apply for an upgrade as late as possible before departure, even though the original ticket has the 30 day limitation). However if your goal is to use the pass to book, but so far in advance, that you can still pick, that have I availability, in order to also use your vouchers, then perhaps the 0 day pass is the wrong choice?

I'm pretty sure any added services resulting in surcharges are not refundable when getting upgraded (seat, more luggage etc.). The undiscounted price for up front seats should be more than that, I think for DUS-ATH 14€ and 16€ (depending on the season), as I gold I'm asked to pay 7€ or 8€ respectively, so 50% discount for up-front, but zero for extra legroom (exit-row seats). If you paid 10€ for AMS, then I'd bet this was the discounted price.

Now a question from me: Can anyone with a pass confirm, that once booked, a flight can be re-booked even outside the original 365-day window? Just try a dummy re-booking and see what dates are available