I am not Mike so these are not definative answers but should be close:
Basing will remain the same. BTW, the game palys fine with any style of basing as long as both armies are based the same. I have used DBX, 20x40mm, 30x60mm, and 20mm square bases as units all with no trouble.
The AP values may change a bit so some units may cost more or less than they used to, debending on their quality and special abilities.
Most of the rules remain the same, we are just adding a few new, fun options.
As for being out by Christmas, probably not for Fantasy. If you want a taste of what the new rules are like MA:Ancients is now available from th eRebel Minis store. We are doing our best to keep the two games compatible, and none of us want to give up our old MA armies.
cheers,
--dave (MA Playtester)