I have seen it posted (not sure if was here or OCB) that the operation of a freezer at fridge temperatures will stuff your freezer fairly quickly.
I went to see a bloke today who is a refrig guru, and posed the question to him. His response was that it was perhaps less than ideal, but that he would not expect it to significantly alter the life of the unit.
If you can find a glass top, why change it?
dreamboat