This article can be helpful when you are trying to change an agent email to an email of a deleted agent.
This error is displayed if the email has been used previously for an agent that has been deleted
Selected email has been used previously. You can not assign the email to this agent.
A) you can't change the email address of an agent to the email address of another agent or an agent that has been deleted.
B) but you can create agent with the same email as an agent that has been deleted. This operation 'undeletes' the deleted agent.
To change email of an existing agent to email of a deleted agent, the required workaround steps are:
1) undelete the deleted agent account john.doe@example.com by creating agent with the same email address. Click on "Create agent" button in Configuration > Agents section
2) change his/hers email address (to something non-existing e.g. john.doe.not.existing@example.com)
3) delete agent john.doe.not.existing@example.com again
4) change the email address of the desired agent to john.doe@example.com