Fine have it your way if you think it's so trivial. I personally believe the meaning of a hero name is really important since it identifies their ideas. So the Le vs Lu is pretty significant imo.
Then point out what is lost in the transition. We all knew it referenced the million. We all knew Mirio is a reference to Tintin. What else is there?
If any given person were to read the manga, what knowledge about the world, character, or anything would they gain with Le that they wouldn't with Lu? Hell, unless they also know Tintin originated in france, they probably wouldn't even make the French connection.
The only thing I can personally think of is that Lemillion rolls off slightly nicer on the tongue than Lumillion. It sounds nicer when spoken. That's it.
I also like to think that Horikoshi's puts lots of thoughts in the name he gives his character, so simply saying it doesn't matter bugs me.
If that were true, Nejire would have a proper hero name /salt