That's a good point. I had thought about leaving it as-is but issuing a warning saying that the environment variable had been replaced with an explicit path. But I think your suggestion is more along the lines of what they'd want to do. If they're using environment variables, they'll likely want to continue. 

So how about if we encounter an environment variable, leave that cfg file alone and issue a warning telling them to update their environment variable, and possibly the file itself if needed  (for cases where the env variable may not be the entire path and they've moved the core to some other root, eg. `/sgtk/studio/$SGTK_CORE_LOCATION` --> `/vfx/sgtk/studio/$SGTK_CORE_LOCATION`).