Hi Ho- we went through this project
a while back. The foam in our refer/freezer was 20 years old, and after digging into it, we found areas that were soggy... no insulation value.
Since you already have the icebox
out, it makes sense to replace the foam. In our case, we used the 'blueboard' sheet insulation from Lowes and Home Depot (one of them had 'pink board'). Before installling the insulation, we lined the area with reflective insulation, and then plastic in which to incapsulate the new insulation. It wasn't a real tough job... the worst of it was removing the cabinetry to get to the refer liner, and then digging out all the old stuff. This is a job you only want to do once, and if you intend to spend time on the boat... having an icebox
that keeps things cold is a bonus. If the icebox meets your needs for now, and you don't expect to increase your time aboard... then you can leave it alone.