Paper ticket seems to have cracked it; I'm just waiting for the confirmation email, but was able to get only second class tickets.
Still a problem with First Class, as it was showing full. 2 of us are on Interrail Passes, but I had already booked 4 seat reservations so we can sit together. Will the other 2 be able to upgrade on the train, since we have seats for them?
Assuming that you are taking EC 57
Wawel, 1st class on that specific train is now fully sold out (only one compartment-type carriage).
Regarding upgrades, it might be a bit messy. DB operate the train west of the border, PKP Intercity crews take over at Rzepin - I don't know if the DB crew will be able to issue an upgrade beyond the border or at all. PKPIC crews can only issue upgrades on domestic sections. All of this is possibly irrelevant as this train has mandatory reservations and a reservation will need to be issued for the upgrade, which may be an issue because there are no unreserved seats left.
Regardless, I suggest speaking to the train manager
before the train leaves - upgrades cannot be issued from a station the train has already departed, and the TM may be lenient with the reservation part if you already have one and it won't be used. You can also try asking someone at the ticket office to see if they can help.