Command Change Custom Event

It would definitely work for both of those (you'd be looking at variant 3, probably, since you'd be affecting commands in different slots). Using Strago with Banon may run into some other issues, though - take a look at the Lore-learning code? You might find that Banon learns Lores even when he can't use them, since IIRC it's linked to the character being in battle, and not the command. Is it possible to make a character in slot 0E or 0F equippable? It's been ages since I've looked at that, and I'm not at my desktop with most of my notes. You might be better off looking at something like that rather than swapping the character, at least in terms of Banon. (Or using another character, like Relm.)

But yeah, this would definitely work for your Gau idea.