vimdiff tips and tricks - how to copy between two screens

do (diff obtain) and dp (diff put) is what you need. Here is a small list of other helpful commands in this context.

]c               - advance to the next block with differences
[c               - reverse search for the previous block with differences
do (diff obtain) - bring changes from the other file to the current file
dp (diff put)    - send changes from the current file to the other file
zo               - unfold/unhide text
zc               - refold/rehide text
zr               - unfold both files completely

Note: Both do and dp work if you are on a block or just one line under a block.

:diffupdate will re-scan the files for changes

Original source:http://unix.stackexchange.com/questions/52754/whats-the-recommended-way-of-copying-changes-with-vimdiff