Hey dude.
I don’t have the answer on how to do this but I came across someone who has done something similar to what you want to do. There is a video which I will post below where the person shows something similar to what you mean but in the description he has posted an image of the blueprint that he has made so that might help you out a little. Just beer in mind that the upgrading city he does is in the editor and not the game but he might be able to help you a little or at least give you a good starting point.
