I know this comes up every year. Maybe I can explain it better, but maybe not.
You can not add dining to an existing package.
Free dining is a new package with a new code.
If the TA does his/her job, you won't even know you've been cancelled and rebooked. Disney transfers the deposit to the new reservation. You will probably be told they have added free dining.
But in reality, your old package has been cancelled and a new one has been booked.
The agent (whether it's AAA, your TA, or Disney) will not be able to add free dining to your package if there is not availability for that room type and those dates.
And in 2007 I changed my existing reservation from All-Star Moives to POP when I added the Free Dining code and once again, they did not cancel me nor change my reservation number.
Now this rarely happens because if you already have a package booked in anticipation of the announcement, you or your agent is going to be on the phones early getting it changed.
But trust me. If you have a package booked during that timeframe and there is no availability for free dining there when you call, they will not be able to "add" free dining to your package.