Hello Jason,
NOV Diagram takes many concepts and implementation details from old Diagram, but at the same time we designed it to be a different product and to accommodate a larger set of features. So porting old code will be straightforward in the following cases:
1. Tools and Batches are virtually the same
2. Layouts are virtually the same
3. Predefined Shapes are virtually the same, except that NOV Diagram will have more Smart Shapes, because it is made with ShapeSheet compatibility in mind.
4. Certain shape aspects are the same
You may need assistance when porting shapes, as the shape architecture in NOV is different. However NOV Diagram shapes are simpler, because many of the features are designed to be implemented via expressions.
So in general it is hard to say whether the porting will be very difficult. It will surely need a complete rewrite though. In any case you should know that we will putting more effort to NOV Diagram in the future, because of its more advanced architecture, cross-platformability and the fact that it is the foundation of Nevron Draw - a commercial competitor of Visio.
We will still be supporting and extending the old Diagram mainly because of web integration, its integration in our reporting products (SSRS, SharePoint), the huge clients base etc. However if you want to use the latest and greatest of Nevron diagramming, we would advice you to migrate to NOV Diagram. In the next release (expected in the next several days) we have added all predefined shapes from the old diagram and in 2016 we are planning on SVG and Autocad exports. This packed with the fact that NOV applications are expected to run on Windows Universal Apps by the middle 2016 gives more than enough reasons to justify the rewrite.
Best Regards,
Nevron Support Team