Pre-release live client's data is currently encrypted and won't be unlocked until May 15th ish.
We also don't have any idea what it's file structure is given it won't let you install it. So we don't even know what bits of it can or cannot be modified to connect to the beta servers until the encryption key gets unlocked. And that's on Blizzard's servers.
Also Beta only has Act 1 while the pre-release client has all the data for all Acts in it.
---------- Post added 2012-04-20 at 08:38 PM ----------
You download the beta client since the full client you get for pre-ordering is encrypted and won't be unlocked until nearer release.