Revision elevated by a gear-shift (6,2)

I believe the answer is:

change up

I'm a little stuck... Click here to teach me more about this clue!

'a gear-shift' is the definition.
I can't judge whether this defines the answer.

'revision elevated by' is the wordplay.
'revision' becomes 'change' (I can't explain this - if you can you should believe this answer much more).
'elevated' becomes 'up' (raised up).
'by' is a charade indicator (letters next to each other).
'change'+'up'='CHANGE-UP'

Can you help me to learn more?

(Another definition for change up that I've seen is " Engage higher gear".)

I've seen this clue in the Sydney Morning Herald.

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