So there are two events here:

Event 1 - Jeff & co leaving.

Event 2 - Demis stepping down. DeepMind doesn't need a "chair" and "Alphabet's chief scientist" is a bullshit title that Jeff invented for himself when he moved from Google Research to GDM, to make it look like we wasn't abandoning the former for the latter.

The third change is basically ratifying the status quo, since Koray has been de factor running GDM for a while now (and directly reporting to Sundar in addition to reporting to Demis, who in turn also reported to Sundar - quite some triangle there).