title_body: Could the d-factor still be changed, to revert to more centralized block production by IOG? Nothing is mentioned anywhere if, and according to which policy, IOG can move the d-“slider” in the other direction, to again start producing more centralized blocks whenever they deem this necessary. Probably a question only Charles or Duncan can answer.
upvoted_answer: Cardano has three different pillars of decentralization:
  The first pillar: block production. Once d hit 0, this pillar was fully decentralized.
  The second pillar: networking. It means how nodes find each other and pass on this information. Currently, this is a topology file and it will be replaced by P2P discovery.
  The third pillar: governance. The way to vote on all protocol parameters (like d) and updates and access fundings of the treasury.
  So to answer your question: yes, theoretically, it would currently be possible to increase the d-parameter again. But should IO do so, it would certainly be a disastrous signal.