You can either run the JQL of the assigned issues you want to reassign and then use the Bulk Edit functionality to change the assignee, or if you did want to use an automation rule (e.g. you may not have the global Make Bulk Changes permission) then you can just as easily do this in an automation rule, which would look something like the following:
(where you would run the JQL within the branch and change Unassigned to the user you wish to reassign them to in the Assign Issues action).
Hope this helps!
Since they have all been already created, Bulk Edit is probably your best option:
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