I still couldn't update or cancel my five day NUA today on one of my modified reservations, so I went ahead and called my ambassador. She understood the problem and that I didn't want to cancel the reservation and start again because the cost in points had gone up. She put me on hold for about ten minutes and then returned and said she had to contact the technical support team to get them released back to my account. The five NUAs showed up instantly in my account, but she told me it could be up to 24 hours for the original reservation to clear the previous applied NUAs from the system and allow me to reapply. While I was still on the phone, the reservation happened to clear, so I reapplied the five NUAs and everything worked perfectly.

In the future, I intend to cancel the NUA before making any modifications to the reservation in the hopes of avoiding this issue.