I'm not sure if your system is different to ours, but I can absolutely bulk transfer process expert from one person to another in the Admin > Users tab, in exactly the same way I can transfer process owners.
After clicking on the Transfer button, the list of processes displayed includes at the end in brackets whether the person is the process owner or expert - see graphic below.
Simply select the processes for which the person is the process expert to transfer these to a new process expert.
After clicking on the Transfer button, the list of processes displayed includes at the end in brackets whether the person is the process owner or expert - see graphic below.
Simply select the processes for which the person is the process expert to transfer these to a new process expert.