Short answer is: nearly all airlines are going to charge you a change fee for date changes, US airlines are not the exception to this. Only the most expensive and unrestricted economy fares (like full Y) might come with a free date change. Discounted economy fares on nearly all airlines come with a change fee.

But here's a possible secret: even on the same airline, the change fee can differ depending on where the ticket was issued. For instance, about 4 years ago I flew United from Singapore to Hong Kong to San Francisco. Then a separate ticket on another airline to my final destination in the USA. But I needed to make a date change on the westbound return TPAC San Francisco to Hong Kong (on United). Because I had bought the ticket online through United's Singapore site (using US bank-issued credit card), the change fee was only the equivalent of US $50. The United phone agent I spoke with commented that wow! if I'd bought in the USA change fee would have been US$150.

Similarly, my TG tickets issued in China (or Thailand) to LAX come with a $75 change fee, whereas the same ticket issued in the USA comes with a fee double that.

I am not sure if this "loophole" has been closed by now, or if it exists on other airlines.