From what I've heard, Paladins and Rangers each get ONE chance to regain their status after they become Fallen, and that's by completing the quests that earn them their respective strongholds. Of course, if you became Fallen by failing a test in Hell, it's a little late to go after your Stronghold.
As far as becoming Evil is concerned, there's no legitimate way to change it back, as Glorfindel said. You can hack it back with Shadowkeeper, but that depends on which version of Shadowkeeper you have. (My version doesn't work with ToB
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