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.
These articles explain more about cancellation policieshttps://support.associationsphere.com/en/support/solutions/articles/67000743536-cancelling-event-levels-and-add-ons
https://support.associationsphere.com/en/support/solutions/articles/67000743537-event-level-and-add-on-cancellations-setup
Internally, you can reassign someone to another recurring event's date or level using the transfer feature in the Tools menu.