For those who have all the extra files, the converter script is in the RPGMakerWeb_plugins folder, in the Shaz subfolder.
If you don't have the extra files yet, you can get it from my pastebin
So I can talk a bit about the converter, but everything else about MV that hasn't yet been announced is still protected by NDA.
The converter is a script that you can put into VX Ace. It is not an MV plug-in.
With the script active, when you playtest the game, it will create an mv-data folder.
It will convert your database and all your maps and events to MV format and save new files to the mv-data folder (it does not modify your Ace project at all).
You can copy all of these files into the data folder of a new MV project and MV will be able to read them.
It will NOT convert resources. You will have to do that manually.
It will NOT convert scripts. You will have to find MV plugin equivalents, see if someone can convert the scripts for you, or use an alternative.
The converted version will include all the tags you have added to notes in the database and maps, but as scripts aren't converted, they will do nothing.
A VX converter is a distinct possibility, but likely will not be available prior to MV's release.
An XP converter is also on my wish list, but it's going to be quite a bit tougher due to the database, tileset & layering differences.
I think I've covered everything, but go ahead and ask if you have any more questions.