The problem arises when the room type you want is fully booked, and that’s the OP’s concern.
If there’s no availability, adding or removing days is a problem. I discussed this with a CM 9 months ahead of time, specifically because my room type was rare (there were only 10 of them) and I was going over a holiday. I asked if I could remove days from the reservation and she said only as long as there was availability. Which didn’t make sense to me.
She said she had to cancel & rebook it, and if mine was the only one left there was a small chance it could get scooped up in that time.
So I made a leading reservation to cover those “iffy” days.
When I did finally get my firm dates, I thought maybe I could cancel that leading reservation and just add to my main one, so I tried to add a day online and couldn’t, though I could book the extra day separately. So I called in and the CM said my original dates were sold out (which I obviously already knew), and to add the extra day he’d have to cancel and rebook. Odds are, no one would nab it in those few minutes but I didn’t want to take the chance.
If there’s availability, you or the CM can modify a reservation and you get the same reservation number, so it doesn’t look like it’s been canceled and rebooked, but it sounds like that’s what they’re really doing. The only way to find that out, though, is to try to modify something that’s completely sold out, which is what happened to me.