I think he has. I know djok fans won't like this, but his motion to me looks more like a club player than a world class professional. For some reason he's started opening the face and straightening his arm behind him. I don't remember that kind of kink in his motion a few years ago..
No reason not to like
I just posted here myself that he has changed it since 2007/8. But to my mind (as a fan), his main problem isn't to do with stamina or posture or action, but purely mental
He's just terrified of failure, and that is making him tentative, nervous, hesitant on his serve (especially the second one which leads to his enormous amount of DFs recently). Just watch how many times he loses serve when trying to serve out a set or match - it's scarily high and has been for over a year now. He is so tight all the time on court - isn't enjoying his tennis at all any more.