You can check your old code into a VCS (e.g. Git, SVN, etc.), then start deleting the parts which you've already converted. The VCS diff tool will tell you clearly what parts have been deleted, acting like this "digital highlighting" approach but simpler (because the old code base will shrink as you convert it).
The most pragmatic approach would be to convert the tests first (if you have any tests...). Then you can write the new code against the tests. If you don't have tests, strongly consider making some. Converting 100KLOC without tests is a recipe for subtle bugs months later.