In reply to valjean:
Yes, I have done this on two pairs with really good results.
Using a razor blade cut the top seam of stiches, then peel back the inner and outer material without cutting it.
Cut the boot to the desired height and chamfer the inside edge.
Glue the outer material back in place, trimming as required. Allow to dry.
Take the liner material over the outer and glue down, trimming as required.
That should give you a neat finish. Mine has lasted 3 season and is still looking good.