When does, if ever, the se change to me for example?
In this context it's always "se", i.e. when it's used with impersonal sentences.
Se has lots of different functions. It is when the pronouns are used as indirect objects or reflexive pronouns that you use all of them.
Me dio un regalo.
He gave me a present.
Le dio un regalo.
He gave him a present.
Me casé con Marta.
I married Marta.
Se casó con Marta.
He married Marta.
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