I agree with the other two answers (by Buffy and jakebeal), but I would also add another possibility that I've seen people make use of. First, note that I don't understand how important this change is, so I'm not necessarily saying you should use this approach — just that it's a possibility.
You can contact the editors even while the paper is out for review to mention that you'll need to make this change. From what you say, it sounds like they'll probably just tell you to wait until you get the paper back to do revisions. It's also possible that they'll forward that information to the reviewer, which may preempt an issue the reviewer planned to raise.
Again, I can't judge whether or not this is the right approach for you, but it has happened to me that the editors of a journal contacted me while I was reviewing a paper to pass along important information from the authors, and it helped with the process. In any case, you really should correct this at some point before publication, for the sake of academic integrity.