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She is not the cheerful woman that she used to be.

= She is not such a cheerful woman as she was.

= She is not what she was ________________ .

She is not what she was _____ . "

Comments  
I cannot think of a way to reasonably do so. Pructus. May I suggest that the most straightforward recasting is 'She is not as cheerful as she was' ?
She is not what she was once, the cheerful woman.
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Why 'the', Anon?
Because I did not read the original sentence with due attention Emotion: wink
Thanks Guru and Anon!

Guru Micawber, then you are giving a hint that, "She is not what she once was, a cheerful woman" is acceptable?
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Other options:
"She is not as cheerful as she used to be."
"She is not the cheerful woman she once was."
She is not what she once was, a cheerful woman
Yes, I think that's OK. Someone may object to the comma. More likely acceptable: She is not what she once was-- a cheerful woman.