Unfortunately it's not up to players, as the playtest continues to make clear - it's entirely up to DMs.
I think it depends on what is being re-fluffed.
WotC and all kinds of other people had made mention in many years in the past the idea that for instance you can fluff your Magic Missiles however you want. The mechanics would remain as 1d4+1 Force damage per missile... but you could make them look like purple lasers, or balls of plasma, or a stream of rubber ducks. Didn't matter. Whatever you wanted to see them as, it was fine. And while the DM could of course argue with you and say "Nuh uh! They don't look like that!"... the player's visualizing the scene however they want, including how their Magic Missiles look. And no amount of DM whining would change that. The player is fluffing their spell in whatever manner they choose.
By the same token... if (general) you as a player visualize in your head how a Channel Divinity looks in-game, and it is different than how you see a Spell looking when it is cast in-game... that's cool. Nice bit of fluff differential on your part. But that is entirely in (general) your own head. You could just as easily be a person for whom the visualization of a Channel and a Spell don't look different at all-- there's like a glow around the Cleric's hands, and then POOF! some effect happens. Yes, the game mechanics of the two would be different (different resource pools for instance), but how they actually are fluffed in the game world does not and need not be different. Or they can be. It's up to the player to decide how they are seeing the effects play out.
Both ways are completely legitimate choices on the part of the player, and the DM has no say in it (or at least no say that the player doesn't GIVE to the DM by hanging their head and sadly agreeing to listen what the DM is demanding these things look like.) And I completely understand that if (general) you as a player have these visualizations of what is a Channel and what is a Spell ingrained in your head... (general) you might have a hard time accepting the move of Divine Smite over to a Spell (not that this even technically applies in this situation, because Divine Smite isn't even a Channel Divinity, it's just a random divine ability the Cleric gets to apply!) But I personally do not believe WotC needs to use (general) your unwillingness to visualize Divine Smite the ability and Divine Smite the Spell as the same kind of effect as a reason for not making this change if they think it's overall worthwhile (if it makes more sense thematically and for ease-of-use by putting all the Smites into one bucket for instance.)
Just because a person can't make the change of seeing Divine Smite the spell the same way they see Divine smite the cleric ability is no reason WotC should necessarily throw out potentially good ideas in my opinion. But who knows? Maybe WotC will see enough resistance to this idea and roll it back? Whatever happens, happens.