What kind of sewing marks are there? Some can be spritzed out with a light spray, some come out with heat, others must be washed but only in cold water... you really need to know what made the marks before doing anything that could possibly set them. It is a lot of work picking out a whole quilt, but hopefully the quilting is not too dense or the stitches too close together. Best would be if the tension was terrible, which would make it easier to pick out. Some people refer to this as "skinning" a quilt, and you can find some possibly useful techniques if you search for this term. Good luck!