Nim isn't a new language, it's been around a while.
I liked playing with Nim in spare time. But, I don't think I'll use it for anything I care about to maintain for a long time.
If you do want the new cooler features, then ya, you may need to port your code but that's the case with any compiler update - you need to run a full regression at the very least.
If that's not what the developers are trying to communicate with a pre-1.0 version I do wonder what they're gating 1.0 on. What needs finalization? Documentation?