If cover is needed somewhere other than where you first posted, you can move a shift to another unit without cancelling it.
Open the shift from Shifts or the Calendar.
Change its unit to the one that needs cover.
Save. If a worker is already booked, they're notified of the change.
Changing the unit is something you can do yourself. Changing the time of a posted shift is the one thing you can't do yourself — see the article on changing the time of a shift.