So I tried again now that I'm in my Express booking window, and the website really was correct about telling me to drop the night in one transaction first, then add the other night after that's complete. Because I was in my Express window I was able to borrow the points I needed to add a night, and then get to the next screen where it told me how many points would be returned for dropping a night (the same number of points), but it does not use those two equivalent numbers of points to cancel each other out - it actually did borrow 16,250 points from my next use year and leave me with 25,500 points in my current use year (where when I started I had 9,250). Functionally, this is not a problem for me because once I sort out which Ocean Ridge reservation(s) I'm canceling, I'll have a pile of points to deposit to 2026 anyway and these will go right along with them. But it is something to keep in mind if you're in a position where you don't want to end up with extra points in your current use year (e.g. after the points deposit deadline for non-VIPs).
So long story short, even though the documentation says "Even better, modifying a reservation to remove nights and add nights can be completed in the same transaction" it's really two transactions completed in one request/process. If all of the points are going to/from the same use year it doesn't matter, but if you're borrowing like I did, you will see the difference.
