..and I'm back, turns out is was a simple procedure...
But yeah I guess instead of actually addressing the question,
I did address your questions. Yes the DM needs to buy into an optional class. Yes, there is a difference between that and extensively re-skinning/re-tooling a class to match a character concept. The former is much quicker & easier for all involved. Also, good classes spark character ideas.
I left the humor under an sblock so as to make the answers easy to see.
hyperbole and sarcasm/humor is the way to further the conversation....
It breaks up the monotony a bit. And it /does/ illustrate that, no, adapting a caster into a non-caster is not nearly as straightforward and simple as saying yes or no to an optional class.
But it's really not an exaggeration, I've seen must simpler issues take much longer to resolve. Gamers can debate. You've participated in the long debates around here, multiple that by ever table to try to adapt an existing class or homebrew one, vs just a simple yes/no to an optional class in print.
In what case it wouldn't work? "Fluffing" something to look different is what I described. You are describing "crunching" which is bending rules to make something different. Re-fluffing is perfectly fine and has absolutely no effect on the game.
even though I'm seriously wondering what makes 5e so much harder to re-skin??
You jumped straight to DM fiat. If you are talking about /just/ re-skinning, then, you need the mechanics to work right for the new fluff. Re-skin magic as not-magic and you've got a non-magical thing that can be dispelled. Change /that/ and you've gone beyond just re-skinning. D&D has never been up for quite that much reskinning, even in 4e, you couldn't re-skin to the point of changing source keywords and the like, not without things getting screwy and needing some rules revision, as well.
It's just the 5e design paradigm, they're not designing anything to be a generic effect that you can just append any fluff to, the process and the result are mixed to a degree, so you do come into all these questions of how & why. If you're not just pushing it to the level of burning the rulebook, that is.
Now, of course, the contrary question: What's the difference between turning down a player who wants to use an optional class, and turning down one who wants to extensively re-skin & re-design an existing one?
The difference between:
"Can I play optional class W from supplement M?" "No."
and
"Can I play a character who's kinda like a bard but not really, and kinda like a Battlemaster but not really, but more a hybrid but not multiclassed per se, and who does some of the cool stuff bards can do but without magic, and three of the not-that-cool things the battlemaster can do but a little better, and some other things neither the bard nor the battlemaster can do, and still without magic?" "Heck No."
What is so difficult about
that?