I currently use Vectorworks 2021 and have this issue with SOME plant symbols. I have a tree that can have a custom spread and height for each instance. Then I have a tree which is "locked" for lack of a better term. It appears that I would have to do as you described and create a new plant to have a different spread and height. Why would this be the case that it works for one and not the other? Having a tree or plant in general with spread and height that can be adjusted in the object info palette would be beneficial for a conceptual plan where we would like to show trees of various sizes without having to place several different species.