It's plausible.
Humans have broken daemonic possession before. Fulgrim was/is a Primarch, and seemingly regarded as pretty remarkable even compared to his brother Primarchs, so if anyone could break daemonic possession, he'd probably be up near the top of the list. He is repeatedly described as being able to quickly master a variety of disciplines, and there is no reason to think that the finer points of chaos sorcery/daemonology would be any different. It's not like he had a hell of a lot else to do.
As to the power of the Daemon who possessed him in the first place - we don't have a really clear idea of how powerful it really was in terms of it's ability to take possession of people. It only managed to take possession of Fulgrim because he was dumb enough to allow it to possess him after it asked him nicely.
Similarly, as to his change of personality - the novel Fulgrim was the story of Fulgrim's descent into madness. He, (and most of his legion) had been pretty much batamphetamine parrot for a long time before the end of the book, to the point that he was murdering people for pretty much no reason, so the fact that it turns out a few books later that he's still an rather unsavoury chap, what-ho old bean?, isn't a huge big surprise.
He might have had a moment where he experienced some regret at the end there, but it was only a moment. The fact that that moment of contrition lead directly to him being possessed by a daemon might also have shifted his perspective a bit too.
