Carlos’ advice is good - if you have the skills and the time, you can certainly use the current engine and later upgrade. There are still a lot of unknowns here.
Converting an app from the old responsive engine to the new one will be a lot of effort. We haven’t converted an entire app ourselves yet so it’s hard to estimate how long it’ll take, but I anticipate it would take at least a couple of weeks of full time work for the average sized app built on Canvas. Once we release a version of the template it’s possible you’d be able to copy over the core pages and reusable elements to your old app, but that would likely cause a lot of issues that won’t have a cookie-cutter solution.
One approach to take would be building the app now on the old responsive engine, and re-building it when the new responsive engine is fully released. Remember that the new engine is still in beta there still may be serious bugs or feature changes that will impact the way that you might want to build your app on the new engine.