If I chose a particular programme now but change my mind at a later date, how flexible is it to change the format of my itinerary?
As there are several modules to your gap year programme it would depend upon what you would like to change. Changes are normally possible up to 12 weeks prior to departure but after that it is quite difficult unless you have a valid reason for wanting to do so. All the gap placements have prior commitments and it is very difficult, especially for the voluntary programmes to actually not turn up at a project where you had committed your time and resources. Most of the volunteer projects go through a lot of effort to actually make you feel special and organize welcome programmes and to drop out last minute on a whim is not advisable or fair.