I had a strange experience amending an ADR a few days ago. I had a Sci Fri res. for our family of 5 but I recently decided that I'd take our 2 year old to the Jo Jo character meal at H&V that day while DH takes the boys to Sci Fi - so I was on to Disney Dining making new ADRs and I made the H&V one for 12.10 (same time as the Sci Fi one) - then I told her I'd like to amend the Sci Fi one to change it from 5 people to 3 people (to open up space for others). She said they can't alter a ressie so I'd have to make a new ADR. I asked if that would be a problem and she said 'no'. I suggested perhaps it was safest to keep the ressie and I'd call the restaurant while we're in WDW and tell them it's changed to 3. She said no, that won't work. So I gave her the ressie number, she cancelled it and then said "oh we have no availability for party of 3". Huh? Didn't I just cancel a party of 5 one? She insisted no availability was showing up so she gave me a ressie for one hour later for DH and the boys.

The part that irritated me really was that she was totally blasé about it and couldn't see the problem and then said "Sci Fi is *very very* popular". Um, yes I know that's why I made that ADR 4 months ago! What I'm going to do is call back at some point and change the H&V one to an hour later too. Anyway, it's no biggie and to be fair she was very friendly and I liked her (for the most part, lol) but it was odd I thought.
OP, I really hope your ADR works out - it's very frustrating when you've been organised enough to make those ADRs early. Good luck!!!