I've (luckily) never had to change the dates for a flight after purchase, but have thought about this issue many times. Let's say that I book a cheap intercontinental ticket that doesn't allow cancellations, but allows changing the dates for a usd180 fee.

How does this work in practice? Do I pay the fee and get to go home earlier or is the price difference always involved. Let's say that I want to fly home earlier and make the decision after the departing flight. The fare for the return price has most likely gone up significantly since the purchase. Do I now pay hundreds of dollars plus the change fee?

This work differently depending on the airline and in some cases depending on the particular. There is something called "fare rules" that explains this but fare rules can be difficult to obtain especially after the tickets have been purchased. You can always contact the airline to receive an explanation.

In general, in addition to the change fee, there may be a fare difference, so ask about both.

Read the fare rules for the specific ticket which you are considering. Those rules will vary by carrier, fare bucket and sometimes by location purchased.

HE CHANGE FEE APPLIES PER TRANSACTION AND PERSON.
A CHANGE IS A ROUTING / OR DATE / OR FLIGHT
MODIFICATION.
CHANGE IS PERMITTED WITHIN TICKET VALIDITY OF
ORIGINAL TICKET.
THE ORIGINAL NON-REFUNDABLE AMOUNT REMAINS
NON-REFUNDABLE.
--------------------------------------------------
WHERE THIS FARE IS COMBINED WITH ANOTHER FARE AND
ONLY ONE FARE COMPONENT IS CHANGED THE PENALTY
CONDITIONS OF THE CHANGED FARE COMPONENT WILL
APPLY.
WHEN MORE THAN ONE FARE COMPONENT IS BEING CHANGED
THE HIGHEST PENALTY OF ALL CHANGED FARE COMPONENTS
WILL APPLY.
--------------------------------------------------
IN CASE OF UPGRADE TO A HIGHER FARE OR IF THE
ITINERARY RESULTS IN A HIGHER FARE THE DIFFERENCE
WILL BE COLLECTED -AND- THE CHANGE FEE WILL BE
APPLIED.

Am I understanding this right that what I was asking about is almost always "upgrade to a higher fare", since you simply can't buy intercontinental tickets a few days before flying at the same fare as weeks in advance?

In this case you will effectively get a credit for the amount you paid, minus the change fee.

If you paid $500 for your current ticket, there is a $200 change fee and the new ticket is $700, you would pay $400 to change, e.g. $500 - $200 + $700 = $400.

Ok, thank you! This is very useful information.

I think you mean --- $700 - $500 + $200 = $400