Calm and serene, instead changed key (6)

I believe the answer is:

sedate

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'calm and serene' is the definition.
(I know that sedate is a more specific form of the attribute calm)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sedate that I've seen before include "Placid" , "Dignified and sombre" , "Calm and dignified in manner" , "Composed and staid" , "Staid and sober" .)

I've seen this clue in the Evening Standard.

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