You actually aren't eligible for downgrade compensation here although you can ask.

In the case of IRROPS you basically have two choices (1) request to stick in your ticketed cabin but know that you may end up flying many hours or even days later when they can next grab a seat in that cabin for you; or (2) accept the next possible set of flights that gets you to your destination but accept you may be downgraded (F --> C+, C+ --> Y, Exit row --> Middle seat, etc.)

By accepting the new set of flights you waive your right to downgrade compensation (it becomes a VDB not IDB) so it's Delta's discretion to give you back miles or throw you miles for the delay.

I get it's annoying but basically once the IRROPs comes up you can either choose to stick to your ticketed cabin but are at the whims of availability to get to your final destination or accept the next set of flights irrespective of the seats available