You can only change them on maps, outside of scenarios, where they are randomized... but in maps with scenarios, you have to find, bring, or dig them up... there, only the cities are fixed. But it will be possible to change them when the patch with the map editor is released...
It would be great if the second patch (which includes the map editor) would be released sooner, otherwise the developers will release it with their tests in August (promised at the end of June):mad: :mad: :mad: