I took a down bag apart and made a quilt. Then I took a synthetic bag apart and made an underquilt. Not sure if it would be worth the effort to make a down bag from a former synthetic bag due to the different type baffling that would be used. Also the syth bag material, if re-used, would leave a lot of thread holes that would potentially be feather loosers down the road. In my thoughts it would be easier to just make a down bag. If you have the down - why waste it on old fabric when new fabric could be used and the sythetic bag could be used for another purpose.

My suggestion ( assuming you no longer want the synthetic bag)- take the synthetic bag apart carefully. Take note of how it is made. This can be a great learning experience. Then use the insulation and materials to remake a quilt for summer use, or hat, or a jacket.