We're working on improving the upgrade functionality. In the meantime if you're having a problem with it, likely the solution is to reconnect the object inside the populator v3 to the object maker input on the populator. Easiest way to do that is go to the new v4 populator, find the "object maker" field, click the button to the right with the green +, go to Assign Object, find the object you want and select it. Now, what I prefer to do is go to the internal network of the old populator, select the object node and Ctrl-X to cut, then go to the internal network of the new populator (right-click and choose Internal Network in the node network view, or open settings for the node and click the gear network next to the node name and choose edit internal network), then Ctrl-V to paste, and finally connect the object output to the Object Maker input at the bottom. The reason I take this approach is then you can delete the old v3 node.
As I said the upgrader will be improved so this kind of workaround shouldn't be necessary in the future.
- Oshyan