I think there should be a button to where you can use the old graphics. This is so people with weaker hardware can still run the game at a playable rate.
I'm not an expert on this.
But I'm pretty sure it technically is not possible to have 2 graphic engines in a game. But the ability to lower graphics settings should definitely be there.
It is possible, but not feasible. It's the same graphics engine anyways, they're not changing the engines as far as I know. This has be done before with the Halo re-releases;
But to answer OP, you should upgrade your PC if you can't run it. Even with the graphical updates LFS isn't very demanding so it's not up to the developers to keep it running on decade old PCs. It just wastes development time for nothing, a PC that can run LFS is quite cheap $100-$200 at most.
Still, this is a new not only graphics, but also kerbs. And this is a very different time on the circle. So, the meaning of the magic "button" is lost. Besides, I don't think you can download hotlaps from the old version.