Every fare is different. It's impossible to know anything based on just "I" or "D". You need to check the detailed fare rules at aa.com, especially the penalties section of each fare.

Generally speaking, a D refundable fare will be changeable/refundable with no fee to a small fee (maybe $50-$100), while an I fare will be non-refundable but changeable with a much higher fee (think $250-$500). But again, without the detailed fare rules, it's impossible to know.