When I close or demolish an upgrade, any ploppable unlocked by that addon is automatically closed as well. Will the same thing happen to other cities in the region? I don't want to accidentally troll other cities if I decided to shut down upgrades I don't need anymore.
OK, since nobody has an answer, I tried it out myself in a private region.
Turning off addons did not affect the other city in the region, although it could be due to the region propagation delay. I'll check again tomorrow and see if anything's changed. I would imagine it's by design so one city shutting down or bulldozing everything doesn't wreak havoc on the region.
Also, only some buildings are shut down when I turn off the prereq. For the Dept of Safety, the Hospital will stay open while Fire and Police stations shut down.
For Dept of Tourism, Landmarks were still unlocked but big parks were not.
Looks like all the specialization unlocks stay after you close the HQs too.
Don't know if these are bugs or not, but right now you can save some $ per hour while keeping the bonus services.
Also, unlocks do not go across the entire region in the 16-city maps. They only go as far as the 4-city subregion.
Wish I could test this from sandbox mode, but everything is unlocked there from the get go.