The x component remains same throughout and the y component decreases(upwards) from the launch point up to the max height and then increases to the original value again(downwards).
So the magnitude of total velocity remains same but the direction changes
But there is a change in momentum during a projectile motion
The change in momentum is given by m(v-u)
initial velocity u= 20sin30(upward).
final velocity v=-20sin 30( negative because it is in thedownward direction)
therefore change in momentum =m(20sin30-(-20sin30))=m(2*20sin30)