Last it was explained to me-- and there's every chance this has changed, even though JFK and LGA are MACs (which is definitley different than co-terminals) -- switching between the 2 still will/could trigger re-issue based on the ticket conditions.
This from some documentation (that, again, may or may not be current):
MAC INFORMATION - Multi-airport city (MAC) information
Co-Terminals
Multi-airport cities are not the same as co-terminals.
Co-terminals are two or more nearby cities, considered the same point for the purpose of fare construction only.
Changing from one co-terminal to another can be rerouting if actually changing from one city to another which would require a reissue.
and
Multi Airport Cities Note - Does not apply to AAdvantage
Co-terminals should not be confused with multi-airport cities. Changes, confirmed or standby, from one of these airports to another airport serving the same city may be permitted.
It is not a change of routing unless fare is airport specific, then passenger may not standby. Add collect to an applicable fare for confirmed changes.
and for NYC:
NYC STANDBY - Tariff - Standby policy for NYC area airports
General Information
Due to the fact that there are multiple airports within the New York/New Jersey area, some passengers may wish to standby between the different airports. Once you have determined that the passenger is eligible to standby you can proceed with the specifics on standby within the New York/New Jersey area airports as outlined below. Remember, these exceptions for NYC airports do not apply to/from Puerto Rico/U.S. Virgin Islands (USVI).
This procedure applies only for travel within the United States or for travel between the United States and Canada. (This includes Web Fares if fare rule allows.)
This does not apply to/from Puerto Rico/U.S. Virgin Islands (USVI).
Application
AA will allow standby from or to any of the NYC airports, provided fare rules and proper routings are observed:
NYC (JFK/LGA)
to
or HPN or
EWR
It is necessary to determine if the fares for each city were common rated (same amount of money) and had the same fare basis code on the date that original ticket was issued.
If both are the same, AA will only charge the change fee and the passenger will be allowed to standby from the other airport.
-passengers may stand by between LGA and JFK at no additional charge as long as there is not a flight application restriction in the fare rule.)