Depending upon how heavily the quilt was originally quilted, removing and replacing squares could be a very time consuming and therefore expensive task. All the quilting stitches would have to be picked out by hand on those blocks, as well as removing the seams of the blocks . Then the new shirt squares would have to be properly prepared/interfaced and hand sewn back into the quilt, and then re quilted to match the rest of the quilt. In my opinion all this would be nearly as much work as making the original quilt. Unless you just have your heart set on having it redone and being prepared to pay the cost, I would suggest you consider having the other 8 t-shirts made into an entirely new quilt.