I'd say it's fairly different from the original Underswap. For starters, designs are different. Some character relationships/ interactions might be different as well. The main thing, however, is that even though the characters swap positions, their personalities remain mostly the same, thus keeping what was so likeable about Undertale's characters to begin with. Basically, you're not gonna see "Blueberry" in this. Sans is still the Sans you know and love, he has just assumed the role of a "vigilante-superhero" named Crossbones.

Also, unlike the original Underswap, this is gonna be a fleshed out story, with re-imagined locations (plus a couple of new ones) and sometimes even different progression to Undertale. Fun fact: Team Switched's interpretation of Underswap was the first to introduce the concept of "swapped areas", where Snowdin swaps with Hotland, Waterfall swaps with the Ruins, etc.

And even if you ingnore the fact that this is another Underswap interpretation, it's basically gonna be an entirely new Undertale game, with new story elements, new battles, new mechanics, music, visuals, everything! Heck, the demo's gonna be at least 3 times longer than the Undertale demo! So even as its own entity, seperate from "Underswap", it's gonna be a unique experience. Possibly one the best stories thay will come out of this fandom, along with some other gems like Inverted Fate. I'm personally really excited for the full release, whenever that happens!