if i bought an r7 tp with a fuji fit on six then changed the shaft to a speeder 757 would the swing weight change dramatically

the swing weight will change, you are moving from a lightweight shaft to a mid weight shaft, the difference between those 2 shafts is rougly 15-20 grams.....swingweight will change about 1-1.5 points but this can easily be offset by putting lighter weights in the head ...2g = 1 swingweight or reducing the length of the 757 shaft by about a quarter inch....every half inch = 3 swing weights