I've seen threads where it was done. I think if you run some searches on TB with various search words you will find some help.
Here's what I recall from memory, and probably not complete.
Remove the case contents, lay the bass in place and create a template, cut styrofoam or other material to fit and glue it lightly in place. You may want a light layer of foam rubber between the back of the instrument and the case. Re-install everything with new fabric, tack-glue in the new fabric. The "Tacky Glue" you find in craft stores (bronze plastic squeeze bottle) actually works pretty well for the fabric. Contact cement will discolor fabric.
You might get away with just tucking in the fabric around the edges of the case, but getting it to fit the depression for the bass body will take time...it will bunch up where it goes into corners.
Try a Google search for "re-do the interior of a guitar case" and you will find some helpful links. I found this one: http://www.ehow.com/how_8696162_glue...itar-case.html