I like a lot of the spell list expansions. They seem generally pretty reasonable. Not a huge fan of letting everyone rework their known spells after every rest. Just kind of cheapens the main attraction of being a Wizard, I think. Though, Warlocks probably do need the help. But mainly I think Warlocks just need 'slightly' more spell slots than we get.
Also, I put this in the thread talking about this UA, but I don't think it cheapens anything the Wizard can do.
First, Bards, Sorcerers and Warlocks only get 2 spells at lvl 1 and 15 at lvl 20. That isn't a lot. Wizards know any 6 at lvl 1, and can prepare up to 6 depending on Int, and they can prepare up to 25 at lvl 20 and can know all of them. Plus having access to every one of their rituals without having to prepare them with their spells book. And they can switch out all of their spells every long rest.
This new UA would give Bards, Sorcerers and Warlocks the ability to switch one of their spells every long rest. It essentially means they "Know" all their spells, like a Cleric or Paladin, but can still only prepare a very, very small number, much less than the Wizard can. And they can only switch out one of them per long rest.
But the fact that they Know all their spells actually makes sense. Sorcerers and Bards (who I always looked at as basically being an alternate Sorcerer, ever since 3.5 made both of them pre-reqs for the Red Dragon Disciple) perform their magic based on instinct, not learning. It would make sense that with a little prep, they'd be able to switch out whatever magic they want. No where near as much flexibility as a Wizard, who has a whole book of spells to choose from and can pick them all willy nilly. But if a Sorcerer knew they were going to need to fly the next day, and spent some time practicing his magic the day before, learning how to change their usual use of magic from speeding people up (Haste) to making them fly (Fly), then they'd be ready to go the next day. The Wizard can have both prepared and still have more spells prepared than the Sorcerer.
And for Warlocks it makes even more sense, because they have a Patron who is above 9th level. They can simply commune with their patron, to learn a different spell, but they can only keep so many in their head at any one time, because it's hard. So when they learn they need to fly, they tell their Patron, and their Patron shows them how to Fly, but can't keep that and Major Illusion in their head. It's why they need to rest so often. Everything is memorized, instead of being on instinct. (I know I've been playing Titus mostly on instinct, but I figure that's just for the first few levels, since he didn't know his Patron.)
It doesn't break anything, but it allows a player to fix a mistaken spell, without having to wait until a lvl up, which takes six months to a year in Play by Post. And it gives them a little more versatility, which they were all sorely lacking.