As the questions pose I have someone who only needs access to the Jira service management as a customer and not the full Jira suite
I'm pretty sure I found that I cannot just deactivate the original Jira User and add them as a customer as that would be a new user, which doesn't have access to the person's old tickets inside the Jira service management.
I hope my question makes sense, I'll happily answer questions.
Cheers,