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 firstname.lastname@example.org 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. email@example.com)
3) delete agent firstname.lastname@example.org again
4) change the email address of the desired agent to email@example.com