OP is simply asking the wrong question and thus got the right answer to the wrong question.

Under AA's current Covid waiver, he may change his ticket with no change fee. If there is a fare difference resulting in an increase, he must pay that. If the fare difference is a decrease, he will receive what AA refers to as a "flight credit" which may be used for future ticket purchases (but is not refundable).

As noted by others, this can generally be accomplished online and the booking tool will display the fare difference to be collected or issued as a "flight credit."

Best to stay away from jargon terms such as "rollover" because it justs shifts the conversation to where it need not go.