I'm not sure that will have the desired effect. You'd likely have to change out all three wheels and tires to make it work. Fat tires have a larger outside diameter to begin with, add larger wheels and the frame of the trike will be jacked up 4 plus inches in the back. Changing geometry of any bike is very tricky to get right and safe. You could make the ride more uncomfortable and dangerous. If you determine the ride isn't what you'd hope, try adjusting air pressure down to minimum specs. Change out the saddle/seat. Add seat-post suspension. Or get slightly larger balloon tires for the stock wheels.