Here’s the big one. All of SRAM’s cable pull ratios have remained the same, so in theory everything should be backwards compatible with old Red, Force and Rival. However, much like Shimano, SRAM insists that the group is designed as a system and therefore will work considerably better with all the correct parts.
The front derailleur in particular benefits from the system approach. Like Shimano, we expect that running full Red will actually improve shift performance dramatically. For the rear, it is much more difficult to say, and we’ll just have to experiment when we get a group back in the U.S.
We can say for certain that old SRAM rings won’t work on the new crankset, though.