Are there actors in the Arrowverse who have previously played DC Comics characters?
Yes, there's lots!
Dr. Christina "Tina" McGee in both the the 1990 version of The Flash
…and the 2014 The Flash series
It's fun seeing her interested in Henry Allen, since her character and John Wesley Shipp's character had romantic feelings in the 1990s show.
Julio Mendez, Barry Allen's coworker and close friend, in the 1990 The Flash TV series.
And Captain Julio Mendez in the "Flashpoint" timeline of the 2014 The Flash:
Superman in Superman Returns
And Ray Palmer (The Atom) in Arrow and Legends of Tomorrow
The voice of Lex Luthor in Superman: The Animated Series and the rest of the DC animated universe (as well as other voice roles)
And General Wade Eiling in the 2014 The Flash
John Wesley Shipp
The Flash himself in the 1990 The Flash series.
And Barry Allen's father, Henry Allen, in the 2014 The Flash, as well as Earth-3 Jay Garrick
As the voice of The Joker in Batman: The Animated Series
And as the Trickster in the 2014 The Flash
In his first episode, he also dramatically tells another guest character "I am your father!"
Voiced Oliver Queen in DC Showcase: Green Arrow, then fought against him as Damien Dahrk in Arrow
He has also done video game voice work as Barry Allen/The Flash in Injustice: Gods Among Us and Floyd Lawton/Deadshot in Batman: Assault on Arkham.
As the voice of Harley Quinn in many Batman-related media:
She has a one-line voiceover role as Harley Quinn in Arrow
"Do you cuties need some counseling? I'm a trained therapist."