It seems that user's account management has changed in Atlassian Cloud. In particular, I'm trying to update the Full Name of a customer's account but the field is grayed out and not available for editing.
There is a current feature request that you can find at CLOUD-9469 for this particular issue. When you have a personal account, only you can update your account details, not the administrator so if the account is a personal account that would explain the behavior you are seeing and is documented in Personal and managed Atlassian accounts. As a workaround you can ask the user to log on to id.atlassian.com and change the name in the Profile.
In order to get more traction on the feature request I would also recomment voting on the issue and adding any comments you think are relevant to the issue.
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