Then please allow me to also add my objection.
This seems like a solution in search of a problem. There is precisely zero benefit to making the proposed rename, and some already drawbacks already acknowledged (however small they may be).
If you wish to remove the useless lower difficulties: great, there's no harm in that. But that doesn't mean that you should also rename the current difficulties. Renaming would not only cause confusion for C2C newbies, as you point out. It would cause confusion for all players, newbies and veterans alike, since this is a new and unprecedented change. It's not just a break with "tradition" – it's a break with a firmly established convention. The only way to be aware of the change is if you happen to read this thread, or the changelog. Sure, it's only a one-time confusion, and easily dispelled. But it's a totally unnecessary confusion, which must be dispelled for each and every player individually.
There are zero drawbacks to retaining the current naming scheme, and therefore no compelling reasons to proceed with a rename.