As mentioned, I'm no D2 expert (I'm the author of UIBUILDER, a rather different approach to data-driven web apps). However, you can have D1 and D2 running in parallel since they both use separate URL's.
The simplest approach therefore would be to live with 2 separate web pages and slowly re-create your D1 stuff in D2 as it becomes feasible.
You can use the same flows as you have been for D1 but, as you add features to your D2, simply take a 2nd output into the D2 node(s) as well as D1 until you are ready to switch over.
