We did new foam about 12years ago. Went with the 'heavy' stuff - it's made a world of difference!
Put the edge of the new foam against your chest and press it together with the heels of your hands. If your hands bottom out, look for different foam.
I took both our old cushions into the upholstery shop under one arm. I carried the new cushions out one at a time. Cost at the time was about $300, reusing our original fabric.