In this sentence,
1 - What are the reasons for using が and は particles in their respective places?
2 - Could we replace any of these two particles with the other one? If yes, how would that change the meaning?
- What are the reasons for using が and は particles in their respective places?
文明が進むと is a subordinate clause (the と-clause) of this sentence. Since 文明 is not the topic of the whole sentence but the subject of a subordinate clause, が imust be used as a simple subject marker. On the other hand, 社会は複雑さを増す is the main clause of this sentence. Since 社会 is the topic of this whole sentence, は has to be used.
The rule of thumb is that you cannot usually use は in subordinate clauses, which includes relative clauses, when-clauses, if-clauses, and so on. (As an exception, a contrastive-wa may appear in subordinate clauses.)
2 - Could we replace any of these two particles with the other one?
No, you can't (unless an explicit contrastive meaning is clearly intended).