I booked the ORD-SEA-PVG, PVG-SEA-ORD roundtrip cash ticket (V->SU) and have flown the ORD-PVG. I am currently sitting on the PVG-ORD return on 11/30, but I need to change it to a later, currently unknown date in december.

I checked the change fee to the same flight on some random dates in december and all of them are showing a $0 difference in fare, which is odd since the tickets have gone up in prices since I booked.

This is good news to me, but if I change it now to some placeholder date in December, will I lose the current no-change-fee ticket when I rebook again?

TIA

You will not lose no change fee, but there's no guarantee flight price will still be same on new flights (due to bucket class inventory availability, seasonality requirements, etc). In particular, you will need to find flights with V bucket inventory since that is what you originally booked. DL currently has no change fees on ex-US Main Cabin or higher fares (when you purchase a roundtrip ex-US, both outbound and return will use US origin fare filings and be subject to same fare rules/change fees). Once outbound has been flown, they can generally use your original ticket issue date to measure advance purchase requirement on fares for return flights (whereas booking a new fare on same flights may not qualify for cheaper advance purchase fares if it is too close-in to meet them) and they can often keep your original fare as long as original booking class inventory is available on new flights (although you may be subject to higher fares due to other issues like seasonality requirements or day of travel requirements on certain cheaper fares). I'm not seeing fares with seasonality requirements right now on cheaper ORD-PVG fares, but V fares for Fri-Mon returns (VNW0ZPMU) are currently $50 higher than Tue-Thu return V fares (VNX0ZPMU).

Also, you will be eligible for roundtrip fares for return flight as it is part of roundtrip itin while attempting to book a new one-way will only allow you to book one-way fares which will generally cost more (although there are some nuances here as attempting to book a new one way ex-China will quote ex-China fares which are different from ex-US fares which is what you are currently booked on).

Thanks for the detailed explanation! I will go on and change with confidence now.

My fare code is VNR0ZPMG/VNR0ZPMG/VNA0ZPMG/VNA0ZPMG, I have no idea how to interpret this though.

You actually only have one fare in each direction (which covers both segments). VNR0ZPMG outbound (valid for FRI-SUN outbound travel) and a slightly cheaper VNA0ZPMG fare for return (valid for TUE-THU return flights). These fares expired on Oct 12th and are only valid for travel from Oct 10th to Dec 10th, so you will not be able to keep return fare if you want to return after the 10th. Fares will be somewhat higher if you change to a return after the 10th as you will no longer qualify for originally booked return fare.