Hi there. I've converted satin to fill on numerous occasions for letters that are just huge. Satin stitching is one stitch from side to side of the column. Fill stitching is more than one stitch side to side, so it will double automatically, and can more than double. I've never had a problem with numerous thread breaks and etc. If you do, you can always lower the density by .5 or so until it looks right. Happy Holidays!