You probably can. Me and a friend both chose the 3 year course and were given the option to change it to 4 year course when we were in year 2, and looking for placements. Similarly, my friend chose the 4 year course but didn’t want to do a placement year so was put onto the 3 year course.
Obviously it changes at different unis but you could ask them and find out, but in second year you’ll probably be given the option and more information on placements then