You can do it without leaving Vim:
:w new_version
(undo until the big change is undone)
:vimdiff new_version
(diffget to get the changes you want)
If there are overlaps it's more work. If there are many changes then
the diff + patch tools might work better, but likely "patch" will find
conflicts and it's not good at dealing with those.
--
hundred-and-one symptoms of being an internet addict:
175. You send yourself e-mail before you go to bed to remind you
what to do when you wake up.
/// Bram Moolenaar -- Br...@Moolenaar.net --
http://www.Moolenaar.net \\\
/// sponsor Vim, vote for features --
http://www.Vim.org/sponsor/ \\\
\\\ an exciting new programming language --
http://www.Zimbu.org ///
\\\ help me help AIDS victims --
http://ICCF-Holland.org ///