Why has he replaced john Cleese?

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Cleese was a very good Q. So were the other actors who came before him.
But, I think Ben Whishaw is great as Q. I like the change to a younger Q.

But the reason "this guy" is playing Q is he's one of the finest actors of his generation.

The Wizard Has Spoken

I thought Cleese was awful as Q,so glad they decided to recast the part. Ben is so much better.

Cleese was the worst Q, Whishaw is a big improvement.

just as the producers and screenwriters reimagined the role of Miss Moneypenny for the 21st century (she's an agent, not a secretary, and NOT panting over Bond), they reimagined the role of Q. The contemporary incarnation of Q, they decided, should be hip young a technology whiz. Ben Whishaw embodies the image of "nerd chic" perfectly.

I loved him in Skyfall, and was delighted to see his character get out of the lab into the field in Spectre. It'd be great to see more of that in the future.

There's also the issue of age. Daniel Craig is well into his 40s, and there have been hints in the last two movies of his getting a bit old for the job. Bringing in significantly younger characters brings that point sharply into focus. In Bond's first meeting with Q at the National Gallery, we see it illustrated brilliantly: first symbolically, as they discuss the painting of the grand old warship being hauled away for scrap (a metaphor for the aging 007), and then openly, with Bond's incredulous observation that "you still have spots". ("Spots" are pimples in British English.) With an actor in his 70s, like Cleese, that scene would have played quite differently.