While you have the old foam out of the cushions, you'll be tempted to wash them. Be careful, as lots of supposedly preshrunk and unshrinkable fabrics will shrink slightly if thoroughly wet and dried. The heat of a dryer sometimes makes it much worse. A small percentage of shrinkage won't matter too much on a small cushion, but it can become a big issue when you've shrunk a 6'+ cushion for a berth.
Don't ask me how I know this....