Okay, so, after a little research, it appears that it's possible to restore back to the original factory state (e.g., the original single partition). Seems as if there's a step if you go that route to repartition, which I'm assuming will do what I want.
So, I'm going to install the preview on the SP3 tomorrow. I did read that Continuum, or the auto-switch from "desktop" to "tablet" modes, won't be in the preview yet, and so I'm not sure how things will be on the SP3--but, there's only one way to find out.
Also, thinking about it, deleting the preview partition and resizing the original partition back to the default shouldn't be too much of a problem even without restoring.