I have opened and saved a project with Delta2D version 3.4 or higher. Can I open it again with version 3.3?

If you have created or modified a project with version 3.4 and higher, this project in general cannot be opened or edited with version 3.3 again. However, you should be able to re-open a project with version 3.3 if there are only approved match vectors in your match maps. But we strongly recommend not to mix using version 3.4 and 3.3 with the same project. In particular editing spots which originally have been detected with version 3.4 and higher will produce unexpected results when you continue spot editing in version 3.3.