If I can sew, you can sew... I learned after finding HF... been hooked ever since...
worst case, go down to your local dry-cleaners, they often to alterations. Have them sew a line 1"-1.5" in from the zipper. Should cost less than $20.
Take it home break out the scissors and cut away. Leave as much raw fabric as possible. Cleanup the down, blow the down out of your nose, and go back. Have them sew a rolled hem on the now open edge and you're done...
"Do or do not, there is no try." -- Yoda