Gas Tank Filler Neck Grommet Change

Should be abke to do it without removing the tank. Pull the rubber cover in the trunk, remove the filler neck, swap grommet and reverse.
At worst you may have to get under the car to R&R the grommet, or loosen the tank straps to lower the tank a bit.
Don't use a jack against the tank though. I always use a chunk of 2x12 or a small pallet to prevent denting the tank.