For our Sept trip I made HDDR ADRs in April for two people. Mid-June my ex decided to join us, so I called, cancelld the first ADR, and rebooked for three people. A few days later I had to change another ADR and while I had the CM on the phone I asked to check all my ADRs. We found the last CM had made HDDR for the first show, instead of the second. She was able to fix it without a problem, so it sounds like they usually have availability.