we amended a rab 400 into a double top bag. The rab had a full length zip which came apart at the bottom like a jacket zip. We wanted to open up the footbox too. As the zip could be opened at the bottom we left that in. We tried to push the down away from the line we wanted to extend along, tacked the cloth together 2" either side of the line to hold most of the down out of the way then cut down the line, carefully, and added extra material to keep full loft. We put the down we found in a paper bag and added a second zip, stuffing the down back in through a small hole before we finished, then unpicked the tacking. We then made a minimal bottom sheet with matching zips and we can use the bag as a single or zip in the bottom sheet to make a double. If your zip does not open at the bottom you need to do similar holding operations with the down and unpick the existing zip and sew in a longer one. It is quite lot of work.