Depends how the engine implements the scaling - if it's tracking rendering load dynamically, and scaling when frametime (or some other metric) hits certain thresholds, it should scale automatically on faster hardware (unless that hardware is doing something to cripple itself to perform like the old hardware for backwards compatibility purposes, like the Wii U booted into Wii mode).
You could also theoretically use some kind of predictive system, using your knowledge of how your engine performs (which on fixed hardware, should be fairly static and easy to predict) to scale based on number of entities/effects/etc on-screen at any one time. In that case, it wouldn't scale automatically. But I don't think I've ever heard of a game that did that (the idtech engines and the Wipeout game on PS3 that had dynamic res all used the former method, I believe).
As for the Forza Apex stuff, I suspect that the engine has been doing that kind of stuff under the covers the whole time (at least to some extent), and that's how Forza has always managed to maintain 60fps so reliably, with Apex (being a PC game, where people expect maximum tweakability) just giving us a glimpse under the hood at the inner workings of the engine, so to speak.
