It depends on the fare rules.

For non-refundable fares you cannot do anything once the departure time has passed; the ticket will have zero value at that point (for all intents and purposes; minor exceptions excluded). Otherwise, as long as you are willing to pay the fees and fare difference you can change it at any point.

For unrestricted tickets there are no restrictions, hence the name.

There is nothing particularly different about international and domestic tickets in this regard. I do not know how the ETSA program fits into this mess for non-US citizens.