Beth, I'm relieved to learn that your itinerary is on one ticket. Like Harold said, your bags will be checked all the way through and you won't have to go through security again. You'll just have to get yourself to the departing gate at Terminal 8 to travel to Milan.
That said, inbound flights to JFK can certainly be delayed for many reasons. If you miss your connection, you will have to wait until the next day assuming there are available seats on that next day flight. You would have to pay for hotel/food as well. I would NOT do this. As was said already, your Milan flight will probably close 30 minutes in advance so you really won't have 90 mins anyway. Probably 60 minutes.
So here is what I would do.
American changed your schedule so I would look online for an earlier flight to JFK or a completely different itinerary to Milan including connecting via Miami. Select 2-3 outbound itineraries that will give you over 2 hours connection time. Then call American Airlines and speak with an agent. I would tell the agent that American changed your itinerary and now you are very uncomfortable with the shorter connection time at JFK. Then tell the agent that you went online and found itineraries with more connection time. He/she might be grateful that you took the time to do this making his/her job easier. If the agent however won't assist you, just say thank you and then end the call. Then call the customer service # back until you find an agent who will help you. Again, this is what I would do.
If your JFK inbound were on time, the new connection time won't be a problem as others have said but if there are delays, your connection time would be too much of a risk at least for me.
So food for thought. Good luck.