Yes, usually. If a passenger can't travel, someone else can take their place, and name changes are free.
Before your tickets are issued: update the passenger's name yourself in Manage booking, using your booking reference, departure date and surname. You can update their dietary requirements there too.
After your tickets are issued: contact the reservations team with the new passenger's name.
The new passenger takes on the existing dietary details unless you tell us otherwise. Travel documents are only valid for the named passengers, so please update the booking rather than handing tickets on.
Related: Can I change my booking?