There were a few during 30K that stayed loyal to the emperor. Nathanial Garro is one and went on to become the first Knight Errant.
40K wise, I think its not likely to happen, Nurgle has infected them so fully that even their soul is devoted to the chaos gods.
I may be wrong but unlike Loyal Space Marines who fall to Chaos, its almost unheard of for Chaos Space Marines to go back to being Loyal.
The closest would be the Alpha Legion, who still see themselves as loyalists to the Imperium of Man (just not the emperor)