It’s very simple, as they are maintaining two different sets of terrain generation algorithms (now doing that is not what I would call “easy”. Not “difficult” either, more like “tedious”, “annoying” or “unsafe”. But I digress…).
If the results of both sets of algorithms is different enough, you don’t have to worry about one of the old planets and one of the new planets having the same seed.
Now, in any sane procgen system it would still be an issue, because the seed would obviously determine which set of algorithms will be used. But structurally NMS at this point seems to be somewhere between the genius of Dr. Frankenstein and the hobbled-together corpse of his creation (Yes, I know, the “monster” is actually good-looking in the book, just really unnerving in an uncany valley kind of way, but you get what I mean).
What it essentially boils down to is this: There is only one way they could leave old planets alone and add the new ones, and that is to complete top-level parameter generation of a system as before without touching anything, and then doing the top-level parameters of the new planets, with the kind of algorithm used not actually being part of those parameters. What you end up with is effectively two separate seeds for two separate procgen systems, and you’re generating half of the system with one, and the other half with the other…
I really, really wish they wouldn’t have done it and just reset the whole thing. It’s going to nip them in the bud at some point in the future, but maybe they plan to seize development before that happens.
Or they just remove the old system at that point and finally go through with the reset…