Does anyone know how I can have a copy of my model in subd or smoothed while my original is is base mesh. I want to make changes on my base mesh while seeing instant update on the subd/smoothed copy.

I saw this somewhere a while ago but just don't remember where. I tried duplicating with instance but it doesn't work. If I convert the duplicate to subd, it separates it from the base (changes don't update). If I smooth the duplicate, then it also smooths the base.

I'll reasearch it after I get out of church today in case nobody knows.