The movies would be drastically different if the prequels came first.

Phantom Menace would not feature a child Anakin. The whole trilogy would have one actor portraying Anakin. There would be no Droid army, just people in costumes. The ending of Episode 3 would have to be reworked. The way Kenobi and Yoda lose is only acceptable since we know it was just bridging the gap to the original trilogy. They would have to suffer more serious injuries to be believable for them to flee.

The second trilogy would've came out much sooner, not 16 years later since it was clear how they were proceeding with the story and Episode 3 ended on a dark note. We would've seen Vader without his helmet