I think it being another version of Gaster makes more sense, and that the Undertale Gaster will stay as is he is, especially because Deltarune is almost definitely an alternate timeline in the same multiverse. There'd still be other versions of Gaster.
We don't know Undertale Gaster's motivation too well, either, albeit he probably wants to reform himself. Honestly, my current theory is that the Mystery Man is actually Deltarune's Gaster invading from another timeline, and when he realizes that he's been noticed, he immediately returns to the Deltarune timeline. Not much evidence to suggest it, but it still sounds cool.
Gaster has fair bit of evidence at least suggesting he's ;potentially the Knight, albeit not too much for that specific theory. I think he could very well be an entirely separate entity, but Gaster is the only character that has any real implications, aside from Kris technically, but Kris is very likely not the original Knight. I'm a bit rusty on this topic but I'll try to recall what I can.
In Deltarune, a man behind a tree gives you an egg. Keep in mind, that man is constantly referred to by terms like "He" and "Him", which are obviously normal pronouns, but are used in a manner most consistent with how they are used for Gaster. When the word egg is translated to wing-dings in all uppercase letters (which is how Gaster speaks in Entry Number 17), the symbols shown can represent the same movements a Knight piece can make in Chess. When Seam is talking about the being that Jevil met, which iirc is implied or even stated to have been the Knight, in the same sequence of dialogue, he says the words "darker, yet darker", which is, of course, linked to Gaster pretty tightly. It is absolutely not a coincidence. Toby is a genius at making these connections, as we've seen in the past, he wouldn't overlook such an obvious connection to Gaster, a character talked about more than almost any other character in Toby's community. While it is fully possible that the being who "helped" Spamton may not be the same as the being that met Jevil, it'd make sense if it was, as they're both apart of the secret bosses group that will continue expanding going forward. Spamton also makes a reference to the words "dark" and "darker" in a manner that also seems reminiscent of Entry Number 17, which, again, we know was made by Gaster. And then there's the whole thing about "garbage noise" which I'm sure you're already aware of. Something few people mention is that the number of Spamton's old friends located near his shop is 6 in specific, a number heavily linked with Gaster, too.
The evidence we have, while very lacking for any definitive conclusions, definitely points to Gaster being the Knight, or the original one, rather, more than any other character I can think of. But personally speaking, my actual belief is that, while Gaster will 100% be relevant in Deltarune, Gaster being the Knight is just a big subversion Toby is planning.
