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You might be able to make the swap with the tires on the ground, but you're more likely to find you want the working space jacking the car will give you.
You will need to raise the end of the spring to relieve the pressure on the mounting bolt. You can use your floor jack for this purpose and you want to ensure the jack won't slip along the spring with the spring under pressure. I used a pair of vise grips on the spring as a stop for the jack.
With the pressure off the end of the spring, it's a matter of holding the head of the spring bolt while loosening the nut.
When I replaced mine i backed it up on some car ranps and used a jack to hold up one end at a time while taking the bolts out. Made it a lot easier to work on things....