Seems like the person in question doesn't have a problem, so I think you should just let things be. Like others said, if you've been asked to teach that class then that decision is for your dojo cho to change or not change, not you. You might feel awkward, well tough. It's good for you.
Try to put yourself in the other persons shoes - he's been away for quite some time and probably just happy to have a chance to train for himself for now! he's already shown by his actions that there's no problem wth you teaching the class.
We have a group of five shodan and nidan in our dojo that alternate teaching the classes our head teacher isn't available. That means that on some particular evenings if I want to train I'm (nidan) going to outrank the person teaching. I keep my mouth shut and train. Actually I tend to talk less to my partners in those classes because I want to be extra supportive of the person teaching. The last thing I would want on an evening like that is to take over the class, since I'm there for my own training.