01 Suburban 1-2 hard shifting ?

Ok I bought a used 2001 4x4 Suburban with the 5.3 engine, the trans shifts hard between 1st and second gears depending on how hard you hit the gas, the dealer gave me paperwork from the previous owner. It shows about a year ago the previous owner payed $800.00 to put in a shift kit and that is why it shifts hard from 1st to 2nd. Does that make sense ? Of course I will be calling the shop that did the work for more details. Am I in for a trans replacement ?