Didn't see anything on this in the annals of the Search box so...
The oft-quoted concept of George Lucas writing the original series with the prequel series already totally in mind is certainly apocryphal.
However, let's imagine that Lucas conceived of what we know as the prequel trilogy first and an analog to "The Phantom Menace" was released in 1977. Followed by an analog to "Attack of the Clones" in 1980 and an analog to "Revenge of the Sith" in 1983.
Your mission:
1. Describe the differences (i.e. casting, plotlines, critical and box office reception, etc.) in a switched prequel trilogy. An emphasis on alternate casting is desired.
2. Describe Episodes Four, Five, and Six (released 1999, 2002, and 2005 respectively) in as much detail as you wish. Again with an emphasis on casting.
3. The outcome historically of these events. (Alternate careers of actors, crew, etc., even alternate history of gaming and fiction if you feel that Star Wars made that profound of an impact on these things.)
Bonus points if Vader being Luke's father is somehow still kept a secret until the analog of "The Empire Strikes Back." (Shouldn't be too hard, really. Adopted name would make a huge difference here.)
Extra bonus points if many of the same "prequel" actors appear in the "OT" and vice versa.
So let's do this!