He isn't such a man as he used to be.
In this sentence, "as" is the correct word to use because it refers back to the type of person he used to be. The phrase "such as" is used to introduce examples or instances of something, but in this context, we want to compare his current state to his previous state. Therefore, "as" is the appropriate choice.
For example:
Both sentences mean that the person has changed from how they were before.
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