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.
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.
And as Charles Hoskinson mentioned several times if the community decides in the future to vote for a particular proposal via Catalyst and the proposal is accepted then will be implemented.
So when Catalyst is fully deployed and we have full decentralization (most probably around or after 2025 based on the roadmap for Voltaire) then the community can vote for this proposal if the proposal is submitted. If for some reason this proposal is accepted (which is highly unlikely) technically can be implemented again.