The HS of the original will be disproportionately big on the smaller body used. SS is using a TBLeague S24A (pale version of the S25B) body for this. You might need to enlarge the neck ball peg (the small S24A body has a smaller neck ball peg compared to standard neck ball pegs) to fit the old head if you want that deformed look.
v1 (original body; no body swap) vs v2
Sideshow Harley Quinn v1 and v2 by
Vicki Lee, on Flickr
Both HS using S25B and S26A body used by SS
Sideshow HQuinn both HS S25B by
Vicki Lee, on Flickr
Comparison using a Katyusha HS on S24/25 bodies
Sideshow HQuinn Unhooded and Katyusha HS_2 S25B by
Vicki Lee, on Flickr
Sideshow HQuinn Unhooded and Katyusha HS_1 S25B by
Vicki Lee, on Flickr
Comparison using S23B (unhooded HS), S24A (hooded HS) and S25B (Katyusha HS)
Sideshow HQuinn S23B S25B and Katyusha S25B by
Vicki Lee, on Flickr
Putting both HS on the smaller TBLeague S26A body (snowflake dress)
Sideshow HQuinn HS_2 S25B and S26A by
Vicki Lee, on Flickr
Sideshow HQuinn HS_1 S25B and S26A by
Vicki Lee, on Flickr
Final verdict: I love the look of the v2 (much closer to the screen appearance compared to v1). However, the biggest disappointment has to be the unhooded HS. It's horrible and I wouldn't recommended it if looking for it parted out. If viewed at other angles, it's just... well... just look at it! If you look closely on the angled shot of the unhooded HS on S26A body, you notice that huge disconnect and unnatural look it gives due to the back of the HS neck going over and swallowing the neck of the body. It's not just an apparent issue with the S26A, it's apparent on ALL the bodies I used in the examples above. It doesn't naturally curve and blend to the neck.