Yes, pronouns referring to a substantive must agree with it. However, you can switch genders if the pronouns no more directly refer to the substantive but to something/someone of a different gender.

So the adjectives must agree with the substantive personne but the remaining clauses aren’t required to agree if you know it’s a man, they only need to agree with the pronoun used.

Unknown (or undisclosed) gender:

Il y a une personne qui est végétarienne et elle est très drôle… mais elle est américaine.

Known gender:

Donald Trump est une personne très intelligente et il est très drôle… mais il est américain.