Can I ask what the difference is in refunds, cancellations, and rebooking policy between these two fare classes?

D (flexible) and I (lowest).

I'm flying CLE-HKG, leaving May 30 and returning Aug 16.

However, I might be accepted into a certain program which would require me to fly into the NJ/NYC area on Jun 15 and spend eight weeks there (So PHL, EWR, JFK, LGA will do).

Unfortunately, I won't get notice on whether I will be accepted until May 27. So I'd be somewhat interested in a flexible ticket, but I have no idea where to start.