Do Chinese airlines provide this for free on domestic flights, specifically on China Eastern and China Southern?

You're basically correct, but it's often possible to change without physically interacting with a human.

Examples:
1. Ctrip (and some other OTAs) can do everything behind the scenes; e.g. if your ticket costs Y600, and the flight you want costs Y700, they simply charge you for the difference plus the change fee, if any

2. If your ticket permits changes, you can change to other flights on the same airline in the same fare class pretty effortlessly, and upfare if needed

3. Full fare tickets are really easy to negotiate with

In all cases, I strongly recommend making changes over the phone or online (I prefer the phone though) rather than in person.

But same-day tickets just hours before departure are usually though of course not always pricey. I'm sure the OP is aware of this and just wants to know if in China he can hop on an earlier flight free.

I agree with this, even though admittedly I have not had many experiences with changing and reissuing airline tickets after making the initial purchase. I learned early from my travels to China that exchanging tickets at the airport is a major PITA. The check-in agents don't handle money transactions, so if you try to check-in to a fight with a different ticket, you will be directed to the ticket counter to have the right one made before you can join the queue again to check-in. My impression is that this ticket policy is fundamentally the same across all intercity modes of public transport in China (air, rail, and bus).

Rail has specific policies dealing with this. You can exchange for free for same day travel on the same route.

The same is more or less true for airplane tickets that are ~6折 and above (cheaper fares are less likely to offer penalty free changes), as long as the same fare class is available; when the fare class in question is sold out, you simply pay a fare difference. Fare class isn't an issue with trains because there is only one price.

I believe this is still true for economy fares, but premium fares were discounted several years ago, and I've seen some crazy numbers (e.g. PEK-XIY for Y6,000). Prior to the deregulation, F was ~30% higher than Y across the board.

From my experience, both DOM Y and J prices have essentially a government controlled maximum price. Recently been looking at HRB-PEK flights around Chinese New Year, and I'm only offered the maximum price (Y is �1290 and J is �5170, pre tax). Plenty of other dates offer much cheaper fares.

I consider the system as basically a price ceiling, from which airlines can discount based on demand.

Prior to business/first deregulation, the cap was around Y1600 for PEK-HRB. I don't believe there are legal caps at all any more; I'm guessing that very few people would, in practice, pay more than $900 for 2 hours in a 737.

ETA: https://www.reuters.com/article/us-c...-idUSKBN1EX0G5

Perhaps, although when I search on Taobao, prices are still expressed as a discount from the full Y/J class fare using an x.x折 format across all airlines, so it's hardly a free market. HRB-PEK has 4-5 airlines on it depending on if the regulators consider HU and CN separate airlines or not, so maybe it's escaping market liberalisation due to being under 5 airlines on the route.

One thing that might also keep airlines honest, particularly on shorter routes, is the spread of HSR in China, which still has set prices. Harbin-Beijing is still uncompetitive, with the fastest service still just over 7 hours (vs 2 hours gate to gate). However from January 1st the Shenyang - Beijing HSR should open and cut at least two hours from that so the train will start looking more attractive. So that should keep airfares under control on this route at least.

I have a feeling that x.x折 pricing will go the way of the dodo bird within a few years when the basis becomes completely meaningless. In other words 50% off only makes sense when the basis is a known constant.

Practically speaking, I can't imagine deregulated pricing having a great deal of impact on most people. Sure, they can charge Y5,000 for PEK-HRB now, but if there are no takers, those fares might as well not exist. What's more, many routes almost never get close to full fare under the current system.

True. I suspect that if it is retained, it'll be mainly for chunyun and possibly the October golden week. The government would probably like to avoid the perception that it'd let airlines gouge people, especially during Chinese New Year.

What the full fare is only matters if you have excess luggage as the cost of excess luggage is a % of the full fare, not what you pay for the ticket.