Do I need to melt old filler before fat grafting?

It is not necessary to remove old filler before fat grafting but you may choose to go this route after consultation with your surgeon.  First, dissolving filler can only be performed with hyaluronic acid fillers so products like Radiesse and Sculptra would not respond.  In that case you would have to wait for these substances to go away naturally or move forward with fat grafting.  If you are having problems with filler under the eyes then it would make sense to dissolve this prior to fat grafting.  Generally there is no correct answer and it really depends on how much filler you currently have, what types, which regions of the face, and how much fat grafting you plan on having done. 

You can either let is dissolve on its own, or try a hyaluronidase to dissolve the material if you want to expedite the process.