Problem Definition
Currently, the REST API only allows for creating new customers.
https://developer.atlassian.com/cloud/jira/service-desk/rest/#api-customer-post

But the endpoint can only be accessed to create a new customer. If there is already inactive user present and if we run the endpoint, an error is thrown stating the user is already created

Suggested Solution
Create a REST API to grant access to portal only customer if the user is inactive

Why this is important
If you have multiple inactive users doing it using REST API will be faster and less expensive than having to edit all customers one by one through the UI

Workaround
Edit on the UI