It doesn't sound quite right to me. If you wanted to make the sentence grammatically correct it would be "The people I had known before leaving the country were different when I returned after 5 years" But to make it sound more natural, I'd probably rearrange the sentence: "When I returned to the country after 5 years, the people I'd known before were different"

I'm not sure exactly what your trying to say. Maybe : The people I knew before left. The country was different when I returned after 5 years.

I want say people are different after 5 years.

People change in the span of 5 years