You shouldn't have a problem. I would start the long arm quilting where you ended the existing quilting and go out rather than quilting towards it. Also, if you are worried about the amount of backing, just take some long fabric scraps, piecing if needed, and using a really long stitch length, baste them to your backing fabric so you have enough to attach it to your frame. I've done this to cheat when I'm a bit short on backing (enough to back the quilt, but not enough to attach to the frame). I have some inexpensive muslin strips that I use for this.