Thanks for contacting us.
It is not possible to unassigned a ticket when customer reply but we have a workaround to fullfill your requirement.
Using the visibility condition for agents, you can allow agents to view and reply to tickets that are assigned to others.
So when a customer replies that ticket will have the status “Awaiting agent reply” so any agent can reply to that ticket.
To set visibility conditions, please go to the Dashboard > Support > Support Agents > Agent Roles > Select Role > Edit > View Tickets/Reply Tickets > select the visibility condition.