It would be in the detailed fare rules specific to the fare you purchased. It is good practice to always save/PDF these when you book a ticket. Here's the relevant language in the "Penalties" section for a domestic ticket I purchased on AA in 2018. It's just one example. I definitely ran into this with AA on an international ticket years ago, trying to change return date/routing on a super-duper-discount-J fare sale, agent wanted to reprice, but I cited the fare rules, and he was then able to change the return.

II. PRIOR TO DEPARTURE OF THE JOURNEY -
CHANGES TO CONTINUING/RETURN FLIGHTS WHEN THERE IS NO CHANGE TO ORIGIN/DESTINATION OR STOPOVERS.
THE FOLLOWING OPTIONS ARE AVAILABLE AND THE LOWEST SOLUTION WILL APPLY.

--- A. ATTEMPT TO KEEP THE SAME FARES AS TICKETED. REPRICE THE ITINERARY USING THE SAME FARE BASIS CODE/ SAME FARE LEVEL/ SAME RULES AS TICKETED/ THAT IS/ THOSE IN EFFECT AT THE TIME THE ORIGINAL TICKET WAS SOLD.
1.NO CHANGE IS PERMITTED TO STOPOVER/ THE FIRST FARE COMPONENT OR FARE BREAKS.
2.THE TICKETED FARE BASIS CODE/SAME RULE MUST BE USED AS THE REPLACEMENT FARE FOR THE REVISED FARE COMPONENT ON THE NEW TICKET.
3.FULLY FLOWN ONE-WAY FARE MUST NOT BE REPRICED AS A HALF ROUND-TRIP FARE.
4.NEW FARE MUST BE EQUAL TO THE PREVIOUS FARE. COMPARE BASE FARE VALUES FOR THE REVISED FARE COMPONENT TO THAT OF THE TICKETED FARE COMPONENT.
5.ALL RULE/BOOKING CODE PROVISIONS MUST BE MET.
6.FLIGHT SPECIFIC FARES MUST REMAIN WITHIN THE TIMES SPECIFIED FOR THAT FARE.
7.ADVANCE RESERVATION REQUIREMENTS ARE MEASURED FROM THE REISSUE DATE TO DEPARTURE OF PRICING UNIT.
8.SERVICE CHARGE APPLIES.

--- B. CANCEL AND START OVER. REPRICE THE ITINEARY USING FARES IN EFFECT ON THE DATE OF NEW TICKET ISSUANCE.
1.USE ANY AA FARE ALLOWED BY COMBINATION AND REISSUE RULES.
2.ALL RULE/BOOKING CODE PROVISIONS OF THE NEW FARE MUST BE MET.
3.ADVANCE RESERVATION REQUIREMENTS ARE MEASURED FROM REISSUE DATE TO DEPARTURE OF THE PRICING UNIT.
4.ANY DIFFERENCE IN FARES PLUS THE SERVICE CHARGE MUST BE COLLECTED. IF THE REPRICE RESULTS IN A LOWER FARE/ THE DIFFERENCE IN FARES LESS THE SERVICE CHARGE MAY BE RETURNED IN THE FORM OF A NONREFUNDABLE TRAVEL VOUCHER VALID FOR TRAVEL VIA AA ONLY

--- C. REPRICE THE ITINERARY USING THE SAME FARE BASIS CODES AND RULES AS THOSE IN EFFECT ON THE DATE OF ORIGINAL TICKET ISSUANCE FOR UNCHANGED FARE COMPONENTS AND USING FARES IN EFFECT ON THE DATE OF NEW TICKET ISSUANCE FOR ALL CHANGED FARE COMPONENTS.
1.NO CHANGE IS PERMITTED TO STOPOVER/ THE FIRST FARE COMPONENT OR FARE BREAKS.
2.THE TICKETED SAME FARE MUST BE USED ON THE FIRST FARE COMPONENT.
3.USE ANY AA FARES ALLOWED BY COMBINATION AND REISSUE RULES.
4.ALL RULE AND BOOKING CODE PROVISIONS MUST BE MET.
5.ADVANCE RESERVATION REQUIREMENTS ARE MEASURED FROM ORIGINAL TICKET DATE TO DEPARTURE OF PRICING UNIT.
6.ANY DIFFERENCE IN FARES PLUS THE SERVICE CHARGE MUST BE COLLECTED. IF THE REPRICE RESULTS IN A LOWER FARE/ THE DIFFERENCE IN FARES LESS THE SERVICE CHARGE MAY BE RETURNED IN THE FORM OF A NONREFUNDABLE TRAVEL VOUCHER VALID FOR TRAVEL VIA AA ONLY.