http://www.omnicorse.it/magazine/54...-ferrari-la-pancia-della-sf15-t-ora-e-svasata
Look at the drop in the sidepod, Ferrari are going mental, changes big and small all over the place. Apparently Arrivabene has said almost everything but the nose has changed in some way or another.
That is some pretty radical changes in the sidepod shape though. Merc have a few little winglet bits around the place, a couple ahead of the sideboard on the side of the cockpit, a couple on the rear crash structure. A cooling inlet along the floor/sidepod join about half way from sidepod entry to back of car maybe. Ferrari look like they've got some vents near the engine also, it's expected to be a pretty hot race/track temp this weekend.
The engine unit thing is pre race weekend. It's effectively how the last race finished.
http://www.fia.com/events/fia-formula-1-world-championship/season-2015/event-timing-information-3
The fia website keeps a page like that for each race, down in the technical reports section you will see a new pdf for, usually one per day, listing which cars took which new parts.
It's been too long from the last race to remember exactly, iirc the biggest issues were all Renault cars had engine issues and Button ers issues, can't recall if anyone else had engine trouble.