Fellwinter's ability to shoulder charge was learned as stated in the fellwinter lore "lady (I think it's jolder) jolder taught me that one" so he was still a warlock. The stranger states "I was not born of light" witch I take to mean that she's not a guardian. Dregon Yor is not truly a guardian anymore. He is now a shadow of someone, controlled by the darkness. Those are the three best Contenders so I don't think so.

Dredgen Yor stopped being a Titan, but he wasn't a Guardian anymore. The Awoken have magic powers, but again, aren't Guardians. Felwinter displayed the ability to shoulder charge, even as a Warlock, but that doesn't make him a different class. - The closest thing to "a different class" that we read about, are the Shadows of Yor - Guardians who use the Darkness in the same way that Dredgen Yor did. This is more likely to be a subclass change rather than a class change however, as they're still what they are originally, just using the Darkness as an element. Hope this helped!