Here's a lesson I learned about insulation. I built a new garage/shop several years ago and insulated the walls with 6" batts. I thought I would buy bags of insulation from the local Big Box Store because the provided a blower free when you bought the insulation from them. But, before doing this, I talked to a local insulation company. They provided two men, one to feed the blower and one to direct the insulation in the attic, and the insulation for a little less than it would have cost for the Big Box deal, plus I didn't have to get someone to feed the blower and I didn't have to get covered with fiberglass. I question the need to remove the old stuff, it is likely doing some good so just beef it up. At least check it out with a professional insulation company.