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Hello, everyone. Help me please.

1)You are not the one who used to be.

2)You are not the one that used to be.

I know that the Relative Pronoun can be substituted with that or when it is in the position of a subject or an object. But in the above, who is used as a complement. When it is used in the complement position, may I use 'that' instead of 'who like the example? I think (2) can be right. Isn't it?
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Your sentences are not complete, there should be an adjective or something after be: "you are not the one who used to be so kind" for example.And even so, it sounds a bit strange. A more natural way would be: "you're no longer the one you used to be", meaning you've changed (for the worst).

As to the relative pronouns, I always favour "who" when it is the subject of the relative clause.
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"You are not who you used to be"

paco