IF you add BEFORE 45 days are up and you've made your final payment...you can add
free dining without paying a change fee.
When there are 45 days or LESS, then you probably would have to pay the change fee of $50. However, I have to tell you, I can only think of ONE time that the free dining offer came out and there were less than 45 days until the start of the dates of free dining. (I think this was 2009.) So as long as you are paying attention you should be able to change your reservation before the dates come out. It IS possible that it might not be available at your preferred resort - in this case, you may also need to change resorts to get FD.
As far as the change, if you are calling Disney Direct, they will not at all be unreceptive to adding free dining. If you use a
Travel Agent, and they refuse to help you get free dining, you've got yourself a really crappy Travel Agent, and I would tell you to go find a new one.