@sedefors you can transfer ownership https://support.atlassian.com/trello/docs/how-to-transfer-boards-to-a-new-account/
If they are no longer going to use their account and do not have any other applications associated with it you could just update the accounts email address with yours. Your email cannot be associated with an atlassian account.
@sedefors can you accept the answer to help others?
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