As you can probably guess, the new rail curves will be incompatible with the old ones. Savegames from 1.1 can be opened and trains will still run on previously built rails just like normal, but you won’t be able to construct the old rails at all anymore. In some future Factorio update when we decide to drop 1.1 savegame compatibility (Let’s say 2.1), we will eventually get rid of the old rail shapes completely.
Ah, I misinterpreted that, it seems you’re right. Still, it looks like there will come a point where 1.1.x maps will no longer be supported, which is again understandable.