Series A processors drained the batteries FAST! We saw those batteries drain within a couple of days when power was off. So they came up with a big, huge, honkin' external battery to help compensate for that, but it wasn't much better. With our recent ice storms around here, there have been places where the power was off for over a week! With Series A processors, I would have had to reload many of those programs.......
Series B processors have internal non-volatile memory. When power is removed, the battery is used only long enough to store the program and user memory to the NVRAM, then the battery is turned off -- it lasts forever this way!
Yes, you can interchange the racks. As TW mentioned, you just have to watch the current loads.