Does the time difference between São Paulo and Seoul change during the year?

No. The gap stays at 12h all year.

When is the best time to call Seoul from São Paulo?

There is no overlap between standard working days, so one side has to call early or late.

São Paulo is 12h behind Seoul. This gap is the same all year — neither city shifts its clocks relative to the other.

São Paulo

Brazil

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Seoul

South Korea

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A 9-to-5 working day in São Paulo does not overlap one in Seoul at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
São Paulo · Brazil	America/Sao_Paulo	UTC−3
Seoul · South Korea	Asia/Seoul	UTC+9

What is the time difference between São Paulo and Seoul?: São Paulo is 12h behind Seoul. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between São Paulo and Seoul change during the year?: No. The gap stays at 12h all year.
When is the best time to call Seoul from São Paulo?: There is no overlap between standard working days, so one side has to call early or late.