I would keep the quilting that was already done, as it has sentimental value. :)
You can add borders to it, but will have to add batting to the existing and whip stitch the batting to the existing batting, the same with backing, you will have to sew a section of backing to each side.
I have done this on a quilt that I was asked to increase in size for someone when they got a bigger bed. I had no problem with adding batting to it and I did the same with the backing, then quilted the addition to the quilt. :)