As long as you do things like this, in off-peak hours, and make very sure you can undo your actions, you'll be  fine.  Fortunately, this issue only involved one file and nothing else.  Sometimes applying changes to fix an issue, involve multiple files, and database updates.  If you have the means, a test site to practice on, can be really helpful... you could set it up as a subdomain on your main site, or just in a separate folder within your sites web root.