SVN (or any similar tool) doesn't distinguish between a modified line and a line that was deleted and replaced with something else. That said, however, your best bet would be to get a diff (as jordan002 said) and then search for lines beginning with + or -. For every -, a line was deleted, for every +, a line was added (you can make svn ignore whitespace on the diff with svn diff --diff-cmd diff -x -w -x -u -rOldRev:NewRev
on a system with Unix diff installed). Then, you can compare them. The unified diff will be organized into hunks (different parts of the file that were changed) separated by lines starting with @@, and you could, for each hunk, say that the lesser of lines deleted and added was the number of lines changed, and for each added line more than removed in a hunk, a line was added, and for each line removed more than added in a hunk, a line was deleted. Not perfect, but relatively good for your assessment.
One more note: if you are using this to evaluate programmers, be careful. LOC isn't a great method of measuring relative effectiveness.