Just read the new variable naming. This is going to create a lot of work. I don't know if anyone is really bothered by having different variable names for different data objects. Not to the point of having to re-code at the level this is going to require.
But if you feel you must do this, then assign the old variables to the new and go through a true deprecation period. In other words, these are marked deprecated in 1.3, but aren't actually pulled until the next release. This will give people enough time to make the change rather than going through what could be a hellishly bad upgrade process as plugins are upgraded at different times after the release.
This is little extra work for you all, but could be the difference between success and failure of the upgrade.
You also might want to tell people to prepare now, by using their own vars.php file, with their own mappings, independent of what you all are doing. This way, code only changes in one spot.