No, volunteers cannot edit the shifts they have signed up for, but you can set up a cancellation policy and allow them to cancel a registration. The advanced portal is required if you would like constituents to be able to cancel their own registrations. Registrations cannot be transferred or edited.
These articles explain more about cancellation policies: https://support.donorview.com/en/support/solutions/articles/9000267268-cancelling-event-levels-and-add-ons