Seems that everyone is confused about this so I will clear everything up.
Prelude is the first chapter of Xsyon and will start at release of game, April 15th, when you will build and discover (for the most part). Your village will be locked from any attacks and everyone can live in relative safety. Prelude will end sometime around mid-end summer.
Before April 15th is the early entry for pre-orders (THIS IS NOT PRELUDE) as a special thanks. on April 15 or slightly before the servers will come down and be wiped.
(possible that only the characters will be wiped so that the towns etc stay in place but It's only a guess)
Hope that clears things up