Several ppl have used a sawzall or a knife, and cut the old bag out. Soak the fasteners and what ever else with a 50/50 mixture of ATF and acetone. The absolute best penetrating agent I have ever used.
Some ppl have purchased a "cheap" wrench or wrench's at Harbor Freight for the bolts on the top of the bag, heated them to an angle to use on those bolts on the FRONT Bags.