Midwesterner, the Genata in Brachos says that a Tzaddik can’t become a Rasha. Then, the Gemara asks your question from Yochanan Cohen Gadol and ends up saying that it is a Machlokes whether it is the same person.
In Yoma 38 the Gemara says that once most of a person’s life went by without an Aveira he won’t go bad. There, the Gemara doesn’t ask anything. I found the Yismach Moshe, referenced in Yoma on Sefaria. The Yismach Moshe is in Re’eh.
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He also answers that Hashem would protect him from a regular Aveira, but if he deviates in Hashkafa then he can still go sour. Similar to a ???? ?? ???????? who loses his earlier merit.