I did have a bit of a chest-thumping moment when I got routers from both versions playing nicely within the same app. Other than that though, it's been fairly devoid of spectacle (just the way I like it.) Most of my complications have arisen from the fact that this was my first project to get my hands dirty with typescript (which, on the whole, I found to be a really nice improvement) and there was an afternoon of headscratching while I was trying to figure out whether to be using observables or promises or both (for good or ill, I'm using both in various places right now.)
My migration path was to take a small new feature and implement it in Angular, but mounted with an angular.js template. That let me prove out the practicality of it on a big-picture basis. The next bit was to implement a larger area of the application and then get both routers working alongside each other. Then some refactoring, moving some of the bits I'd just finished into their own modules (before this, I was putting everything into the application module, which was getting crowded fast.)
So now I have new routes in the submodules and I'm just mounting new features that way. We're converting other modules as time permits but I suspect we'll have both coexisting alongside each other for quite a while yet. As long as maintenance on the older parts of the app is pretty limited, I don't really think its too bad. This isn't an app that's a labor of love or anything—the old stuff is working ok, and when something comes up that demands a lot of work in there, we'll convert it at that point.