It's not so much a change in the class mechanics (though I would still argue it is a big change).
The main thing here is that it changes the identity of the character. A character who has few known spells and can only change 1 every level up has a lot of their identity tied in with those spells. The spells they can cast has a huge reflection on who they are. Change them every long rest and that is no longer the case.
We lose a tool for character identity.
There is nothing in the new rule that says you must change a spell every long rest. If this goes to print, it will be an optional rule. You don't have to follow it. Even if it becomes a hard and fast rule, and is retconned into the PHB, and enforced by WOTC police coming to your game and forcing the DM at gunpoint to allow it in his game, you, the player, still do not need to utilize it if you want your Sorcerer to be a Fire Sorcerer that only uses Fire attacks because that is your character concept. Identity preserved. Literally nothing changes.
But my Sorcerer is a Divine Soul who is able to tap into the vast cosmic powers of his Angelic Ancestry, and therefore my sorcerer is able to, instinctively, cast any spell on either list, but his human side limits how much he can hold at one time. Therefore, if he wants to learn a new spell, he must take a long rest and contemplate how to utilize that energy in a different way, thus 'forgetting' the old spell and 'learning' the new one.
My Warlock is buddy buddy with his Patron, and likes being prepared like a Wizard, but got kicked out of the Wizard college for sleeping with the Headmaster's daughter. He went to the Patron to learn, and took the Tome for extra cantrips and rituals, and he prepares for his day by looking through which spells his Patron knows, since his Patron knows all of them, and deciding which would work best for the day. His whole spiel is that he's a versatile caster, able to do all kinds of magic, but is limited because he only has 2 slots a short rest, so he does what he can to be versatile without being a Wizard. Invocations would likely be ones that allow for at will casting of spells, for more versatility.
My Bard is a dabbler in magic, and has been a book worm his whole life. His understanding of lore and magical history is vast, and he knows all kinds of things that most Wizards wouldn't even remember. Granted it's because most of it is useless, but his studies have given him a great insight on how spells work. Using his artistic ability, he is able to work out how to do the different spells on his list, but like the Sorcerer he is only human, and can only keep so much in his head. Thus, he can switch out a spell at a long rest.
Nothing is lost in Identity. The Identity of the character is in how you play him, not what class mechanics they use. If you want to use more difficult rules because it fits your character, then more power to you! Another Sorcerer concept I've used (that was amazing, by the way) treats the Sorcerer Class more like the X-Men, where all the spells follow a specific theme. No spell changes needed. Like my Human Dragonic Sorcerer. I took spells that made him like a Dragon. Darkvision, Enlarge/Reduce, Dragon Breath, Fly, that kind of thing. It was awesome. He wouldn't benefit from switching spells every long rest, because there are only a few spells that make me feel like I'm accessing his Draconic heritage. But I'm not going to say the class shouldn't be able to, just because it doesn't fit one kind of character concept.
It's not always about accidentally picking bad spells, either. It's about being forced to pick spells before you know what situations you'll be in, but only having an extremely limited selection, and then being stuck until you level up. Wizards don't have that problem, they get spells aplenty. You turn lvl 5 Sorcerer, and you think "Oh, Fireball! And Fly, yes!" And then you go underground in a flooded cavern and have to bum a Waterbreathing Spell from an ally, and now your fireball doesn't work and Fly is useless. Your third level spell picks are shot to hell and you feel like a dope because you didn't know what was going to happen. And those are two of the six spells you even know at this point, whereas the Wizard's got 14 spells in his book, at least 9 of which are prepared and the rest are likely rituals. Yeah, the Sorcerer gets meta magic. So he can subtly cast his useless Fireball. With Spell Versatility, you can be like "Ok, cool, let's sleep on it, and I'll switch Fireball for Lightning Bolt." Or if you specifically have a fire motif, you can switch Fly for something that will help with underwater stuff.
I'm just saying, the "identity" argument doesn't fly, because there are plenty of character concepts that can fit being able to switch out a single spell every day. And it doesn't step on the Wizard's motifs any more than the Cleric, Druid, or Paladin do.
dnd.wizards.com
