It would help if you could post a picture. In general, I would say it really depends on the existing quilting & what you plan to do with it.
If the hand stitches are really quite even, small & straight, it would make it much easier to combine with machine quilting.
I am working on a quilt now that I mostly machine quilted, but then wanted to add some additional micro-filler inside some of the applique designs . Because I am not especially skilled at quilting on less than a 3/4" scale and I had already added the baby rick-rack, it was not practical to machine quilt, so I hand quilted those areas. From a couple feet away, you can't tell the difference. Up close, they definitely don't look the same if you're trying to look for inconsistencies (plus, this was the first time I've ever hand quilted anything ... so there's that). Some of my hand stitches went slightly off-line. Some were crooked. And you will pretty much always have wider spacing between stitches with hand quilting vs. machine quilting.
Nevertheless, unless the hand quilting was really sub-par or just way, way better than what I could do on a machine, I would probably just go ahead and FMQ the rest of it if it were for my family. "Done is better than perfect", as the saying goes.
