Characters that were drastically changed for the better

[Flash Gordon]

Dolphin when she was reintroduced in "Rebirth."

I realise she didn't start off as an "Aquaman" character but now that she's well-established as a part of the Aqua-Family, her current origin as a Sea-changed Atlantean fits far better I think. "Human abducted by aliens and altered into a pseudo-Atlantean" seems contrived to me, even by comic standards, while not really making any practical difference. Sea-changed Atlantean is far more streamlined while also making her more distinctive in terms of her powers/appearance.

Plus bringing back her muteness. That was excellent.

This one, big time. Rebirth also made her into a character as opposed to the object she so often was in the 90s 'Savage Aquaman' books.

Always thought she had potential, just didn't like her being paired off with Aquaman...and Aqualad. She works really well, now.

[scilover]

I think Jason Todd turning into the Red Hood is the best change ever. From a robin under Batman's wing, he changed into a badass gun-using killer anti-hero called the Red Hood!

[Slim Shady]

Most of mine have been mentioned.

So kind of on a a smaller scale, Kite-Man. I hated King’s Batman run but he did turn Kite-Man from a forgotten, cheesy, aged, gimmicky character into a sympathetic underdog type with a little depth.

[Gaius]

Not so sure if it’s much of a reinvention but The Spectre going from just a guy with vague ghost/spiritual powers to the embodiment of God’s Vengeance.

[Sutekh]

I think Jason Todd turning into the Red Hood is the best change ever. From a robin under Batman's wing, he changed into a badass gun-using killer anti-hero called the Red Hood!

That was a great example of 'no such thing as a bad character, just bad writing.'

There was a guy that pretty much everyone had given up on, or regarded as a mistake, and bang, dusted off, given a *great* compelling storyline, and suddenly shiny and new and interesting!

[Masterff]

Actually Bizarro was very cool in NEW52...

Characters who were changed for the better:

Conner Kent from YJ to TT
Bart Allen from YJ to TT
Cassie from YJ to TT
Tim Drake from YJ to TT

Garth from TT to Titans (MUCH better look etc.)
Dick from Robin to Nightwing
Jason from Robin to Red Hood
Arsenal from Red Arrow to Arsenal
Donna from Wonder Girl to Donna Troy

[SuperX]

Actually Bizarro was very cool in NEW52...

Characters who were changed for the better:

Conner Kent from YJ to TT
Bart Allen from YJ to TT
Cassie from YJ to TT
Tim Drake from YJ to TT

Garth from TT to Titans (MUCH better look etc.)
Dick from Robin to Nightwing
Jason from Robin to Red Hood
Arsenal from Red Arrow to Arsenal
Donna from Wonder Girl to Donna Troy

Disagree with the YJ characters, but totally agree with your Titans characters

[Adekis]

I'd argue in favor Jason Todd, not going from Robin to Red Hood, but for his change from Ruthless Well-Intentioned Villain Red Hood, to Edgy Bad Boy Hero Red Hood. Though I do think it's funny that Roy still occasionally gives him flack for that "human heads in a duffel bag" incident. He's still someone whose done murders, but he's not a villain, he's just a different kind of hero.

As for Blue Beetle and Question, I think the decision to turn them into an insecure joke and an acid guru were both mistakes that watered down what was unique about them as Ditko characters in the first place, but who asked me?

[Manakel]

Steve Trevor, from dude in distress bland character from a character actually having a personality since N52

[Frontier]

Actually Bizarro was very cool in NEW52...

Which Bizarro are we talking about? The clone in Forever Evil or Bizarroworld Bizarro who appeared in only one issue?