JJeffrey gave good answers. Allow me to add a few things.

1. If the change is something you want/need to do, changing an awards itinerary involves canceling the existing itinerary online. The miles are reinstated quickly and without any fee. You would then make a new points reservation for the new itinerary, very possibly requiring more points. If there's a major schedule change (greater than 4 hours) that doesn't work for you, then an AA phone agent can move you to flights that work better for you, even if those flights would require more points. (I just did this for an American Airlines flight, not for a partner flight.)

2. If you're able to plan far in advance (around 11 months), points requirements can sometimes be much lower or there can be availability for flights that will soon become unavailable. This is especially true for American Airlines flights, but I've also seen this with partner flights.

3. Itineraries with stopovers (such as spending a few days in a city where you would connect anyway) are treated as separate flights. No free stopovers. There may be cases when a connection is very long (such as when the second segment does not depart until the following day), in which case you could think of it as a short stopover on a single ticket.