Alan, when you think about the change in the maps remember there are two basic possibilities. You refer to one which is a change in conditions (downstream), but a change in the knowledge of conditions will also support a change in the maps.
You might ask the engineering firm to specify the basis for moving the flood plane boundary. Some engineers are so conservative they could not function in a real job that requires practical solutions. But don't insult them or they will lock in their opinion as a matter of ego.
The more informed and professional you are about addressing your predicament, the better chance of getting some relief. Also, if someone has changed the downstream watershed that is causing the change in your flood risk, you may have a case against that person.
There is a Corp of Engineers reservoir downstream only about 4 miles, there since 1986. The previous maps were done since then, and nothing has happened since then. The engineering firm won't get specific other than they "ran the numbers" and this is what they came up with. And that I could pay them to evaluate my particular property which has previously been safe according to FEMA. In other words , they flooded a bunch of us, and for a fee they will come in and rescue us...maybe.