Is it possible for me to make a default city into a region? I have a colony I started before regions were a thing but now I wanna use regions without starting over. So can I put my existing city into a new region?
Unfortunately that's not possible at the moment.
But you can create a Region and send resources from your other, already existing, colony to make a quick start. Also if you want to know how exactly Regions work, just check out the Encyclopedia or go back to the Release Note of the version in which the Region Map was first announced.
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Yeah I though this would be the case, maybe a future game feature?