The transition didn't seem convincing to me at all. For the first part of Kovu's adult life, every part of him we see makes look like a suave, brave, cool-headed bad guy. He rushes into a raging fire to rescue Kiara for the sole reason of earning Simba's trust, then solidly stands his ground as he calmly requests to join Simba's pride. As he follows Simba and the others back to Pride Rock, he even gives that sly grin as if to say to the audience, "Yeah, I'm a bad guy, and don't you forget it".
However, the Kovu after the rhino chase seems like a completely different lion. He becomes a mopey, emotional softy. Gone is his sly, cool demeanor and the witty banter he shared with Kiara. Suddenly, the lion trained all his life to be a powerful fighter gets knocked out with just one kick from his sister as he's trying to save Simba, he just stands on top of the gorge and
looks at Simba as Simba is trying to escape the Outlanders, and
the way he cowers under Zira's gaze -- I'm sorry, I can't think of any other way to describe it -- makes him look like a complete wuss.
And after running away from
her, he then runs away from a mass of animals trying to force him away from the lioness he's just fallen in love with, not even trying to fight them back in an attempt to explain what really happened during the ambush. We see a little glimmer of his old self when he's standing up to his mother in the final battle (but probably only because he has Kiara with him), and when Kiara falls into the gorge, he doesn't try to save her like he did in the fire, he just stands on top of the gorge and yells her name.
Yes, it's definitely possible that Kovu had some emotional issues as an adult
before his transition, but without actually seeing evidence of them, it just makes his transition that much more jarring.
Sorry about the rant -- I just figured that since there was a thread about this topic going on, I might as well say my piece.