I've actually been doing this for ages. I'm reasonable at sewing (not like my award-winning, quilter mom) but I get by.
I've taken pretty velvet tops, added a skirt to them, and had gorgeous dresses.
I've changed necklines on a million things. I hate turtlenecks, and I've redone a gazillion tops to have a neck I can tolerate.
I've taken a beat up dress, a beat up pair of jeans, and cut the jeans for the "yoke" of a skirt, added, the skirt from the dress, and had a cool hippie skirt.
Recently I've been altering all my jeans like mad, adding trim and embellishment. I decided I was done with boring jeans. (grin)